Compositions and methods for producing tobacco plants and products having altered alkaloid levels with desirable leaf quality

ABSTRACT

The present disclosure includes methods and compositions for improving leaf quality in low-alkaloid tobacco plants, e.g., by combining inducible promoters and non-coding RNAs for suppression of an ornithine decarboxylase (ODC) gene. Also provided are low alkaloid tobacco plants with normal, suppressed, or otherwise altered polyamine levels. Further provided are tobacco plants with altered total alkaloid, nicotine levels, commercially acceptable leaf grade, their development via breeding or transgenic approaches, and production of tobacco products from these tobacco plants.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No. 16/291,878, filed Mar. 4, 2019 (now U.S. Pat. No. 11,220,695), which claims priority to U.S. Provisional Application No. 62/638,928, filed on Mar. 5, 2018, both of which are incorporated by reference herein in their entireties.

INCORPORATION OF SEQUENCE LISTING

A sequence listing contained in the ASCII file named “P34584US02_SL.txt” which is 88,060 bytes (measured in MS-Windows®) and created on Dec. 17, 2021, is filed electronically herewith and incorporated by reference in its entirety.

FIELD

The present disclosure includes tobacco plants having altered total alkaloid and nicotine levels and commercially acceptable leaf grade, their development via breeding or transgenic approaches, and production of tobacco products from these tobacco plants.

BACKGROUND

Tobacco is one of the most widely grown non-food crops in the world with global production exceeding 7.4 million tons (FAOSTAT, Food and Agriculture Organization of the United Nations (FAO) (2014), faostat.fao.org) and resulting tobacco products having an annual global market size of USD 770 billion (Euromonitor International, 2016). Nicotine is the main alkaloid accumulating in tobacco leaves. Nicotine and other minor alkaloids are also precursors to tobacco-specific nitrosamines (TSNA). Demands exist for development of tobacco cultivars with lower levels of nicotine.

In commercial tobacco cultivars, nicotine represents 90-95% of the total alkaloid pool or 2-5% of total leaf dry weight (Saitoh F, Nona M, Kawashima N (1985). The alkaloid contents of sixty Nicotiana species. Phytochem. 24: 477-480). Nicotine is synthesized in the roots (Dawson RF (1942) Accumulation of nicotine in reciprocal grafts of tomato and tobacco. Am. J. Bot. 29: 66-71), and translocated through the xylem (Baldwin IT (1988). The alkaloidal responses of wild tobacco to real and simulated herbivory. Oecologia 77: 378-381) to aerial parts of the plant (Hildreth S B, Gehman E A, Yang H, Lu R H, Ritesh K C, Harich K C, Yu S, Lin J, Sandoe J L, Okumoto S, Murphyd, A S, Jeleskoaet J G (2011). Tobacco nicotine uptake permease (NUP1) affects alkaloid metabolism. Proc. Natl. Acad. Sci. USA 108: 18179-18184) where it accumulates in the leaves and is exuded by trichomes in response to insect herbivory (Kessler A, Baldwin IT (2002). Plant responses to insect herbivory: the emerging molecular analysis. Annu Rev. Plant Biol. 53: 299-328). Nicotine biosynthesis is influenced by genetic factors, plant development, biotic and abiotic stresses, phytohormonal signals and agronomic management practices such as topping and suckering (Wang S S, Shi Q M, Li W Q, Niu J F, Li C J, Zhang F S (2008). Nicotine concentration in leaves of flue-cured tobacco plants as affected by removal of the shoot apex and lateral buds. J. Integr. Plant Bio. 50: 958-964; Shoji T, Hashimoto T (2015). Stress-induced expression of NICOTINE2-locus genes and their homologs encoding Ethylene Response Factor transcription factors in tobacco. Phytochem. 113: 41-49). The genetic regulation of nicotine biosynthesis correlates to two independent loci, Nic1 and Nic2, which have a synergistic effect on nicotine levels, but the effect of Nic1 is ˜2.4 times stronger than that of Nic2 (Legg P D, Collins G B (1971). Inheritance of percent total alkaloids in Nicotiana tabacum L. II. Genetic effects of two loci in Burley 21×LA Burley 21 populations. Can. J. Genet. Cytol. 13: 287-291). Both loci also influence the expression of numerous other genes unrelated to the nicotine biosynthesis pathway (Kidd S K, Melillo A A, Lu R H, Reed D G, Kuno N, Uchida K, Furuya M, Jelesko J G (2006). The A and B loci in tobacco regulate a network of stress response genes, few of which are associated with nicotine biosynthesis. Plant Mol. Biol. 60: 699-716; Shoji T, Kajikawa M, Hashimoto T (2010). Clustered transcription factor genes regulate nicotine biosynthesis in tobacco. Plant Cell 22: 3390-3409). Transcriptional analysis has shown that the Nic2 locus is a gene cluster that encodes at least seven ethylene response transcription factors (ERFs) (Shoji et al. 2010).

Homozygous mutations of either one or both loci can be used to create near-isogenic Burley 21 lines with reduced alkaloid levels, i.e. a high-intermediate (HI) variety with the genotype nic2, a low-intermediate (LI) variety with the genotype nic1, and a low-alkaloid (LA) variety with the genotype nic1nic2 (Legg P D, Chaplin J F, Collins G B (1969). Inheritance of percent total alkaloids in Nicotiana tabacum L. J. Hered. 60: 213-217; Legg et al. 1971). LA Burley 21 plants contain only ˜5.7% of the total alkaloid levels found in the normal-alkaloid (NA) wild-type variety (Legg P D, Collins G B, Littion C C (1970). Registration of LA Burley 21 tobacco germplasm. Crop. Sci. 10: 212). In LA plants, the synergistic effect of the nic1 and nic2 mutations also causes an unfavorable leaf phenotype characterized by lower yields, delayed ripening and senescence, higher susceptibility to insect herbivory, and poor end-product quality after curing (Chaplin J F, Weeks W W (1976). Association between percent total alkaloids and other traits in flue-cured tobacco. Crop Sci. 16: 416-418; Legg et al. 1970; Chaplin J F, Burk L G (1983). Agronomic, chemical, and smoke characteristics of flue-cured tobacco lines with different levels of total alkaloids. Crop Sci. 75: 133-136).

There is a need to identify genes that restore unfavorable leaf phenotypes in the LA variety of tobacco plants, and to develop tobacco plants and products that contain altered nicotine levels (e.g., reduced nicotine) while maintaining (if not making superior) tobacco leaf quality.

SUMMARY

In an aspect, the present disclosure provide a tobacco plant comprising an inducible promoter operably linked to a transcribable DNA sequence encoding a non-coding RNA for suppression of an ornithine decarboxylase (ODC) gene.

In another aspect, the present disclosure provide a tobacco plant, or part thereof, comprising relative to a control tobacco plant: a first genome modification providing a lower level of nicotine or total alkaloid, and a second genome modification providing a comparable level of one or more traits selected from the group consisting of total leaf polyamine level, total root polyamine level, total leaf chlorophyll level, mesophyll cell number per leaf area unit, and leaf epidermal cell size; and wherein said control plant does not have both said first and said second genome modifications.

In an aspect, the present disclosure provide a method for improving leaf quality in a reduced-alkaloid tobacco plant, said method comprising: growing a tobacco plant, reducing the level of putrescine in said tobacco plant, harvesting leaves from said tobacco plant.

In another aspect, the present disclosure provide a method for improving leaf quality in a reduced-alkaloid tobacco plant, said method comprising: growing a tobacco plant, suppressing the expression or activity of an ornithine decarboxylase (ODC) gene in said tobacco plant, harvesting leaves from said tobacco plant

In an aspect, a tobacco plant is provided having suppressed MYB8 activity via either transgene suppression, mutagenesis, or targeted genome editing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: Phenotypic characterization of N. tabacum L. cv. Burley 21 normal-alkaloid (NA) wild-type plants and three mutant varieties grown in the greenhouse. FIG. 1A: Total chlorophyll content of wild-type (NA) high-intermediate (HI, nic2), low-intermediate (LI, nic1) and low-alkaloid (LA, nic1nic2) lines grown in the greenhouse. Chlorophyll was measured twice per leaf in all leaves longer than 15 cm at different developmental stages: before flowering (2.5 weeks before topping), at topping, 1 week post topping (WPT), 2.5 WPT and harvest. Values are means of four biological replicates. FIG. 1B: Representative photos of NA and LA plants at harvest. FIG. 1C: Time-course evaluation of mesophyll cell number in leaf 15. FIG. 1D: Microscopic images of mesophyll cells of leaf 15 from NA and LA plants at different plant developmental stages. Bar=100 μm. Values in A and C are means of six biological replicates. Error bars represent standard deviations of the mean. Statistical difference to NA is shown: *p<0.05.

FIG. 2: Polyamine analysis in field and greenhouse grown NA and LA plants. FIG. 2A: Free (F) and conjugated (C) putrescine (Put), spermidine (Spd) and spermine (Spm) content from five well-expanded upper leaves of NA and LA plants (three biological replicates grown in the field) 1 week after topping. FIG. 2B: Time-course monitoring of total polyamine content in NA and LA plants grown in the greenhouse. Leaf samples were collected from leaves at the same stage of development: before flowering (leaf 12), at topping (leaf 19) and at harvest leaf (24). Root samples were collected at topping and harvest. Values are means of three (A)/four (B) biological replicates. Error bars represent standard deviations of the mean. Statistical difference to NA is shown: *p<0.05; **p<0.005. PA: polyamine; FW: fresh weight.

FIG. 3: Polyamine content in the leaves and roots of NA, HI, LI and LA plants grown in the greenhouse. Free (F) and conjugated (C) putrescine (Put), spermidine (Spd) and spermine (Spm) fractions in leaves (FIG. 3A) and roots (FIG. 3B) of NA, HI, LI and LA plants before flowering (leaf 6), at topping (young leaf 23, roots) and at harvest (matured leaf 23, roots) are shown. Samples were collected 4 h after illumination, frozen immediately in liquid nitrogen and analyzed by LC-MS/MS. Values are means of three biological replicates. Error bars represent standard deviations of the mean. Statistical difference is shown: *p<0.05; **p<0.001, indicating that the LI and LA plants are significantly different from NA plants under the same conditions. Only samples from topping and harvest were available for roots. FW: fresh weight.

FIG. 4: Activity of polyamine biosynthesis enzymes. Analysis of arginine decarboxylase (ADC) (FIG. 4A) and ornithine decarboxylase (ODC) (FIG. 4B) activity in leaves and roots of NA and LA plants at topping (young leaf 23, roots) and harvest (matured leaf 23, roots). Values are means of three biological replicates. Error bars represent standard deviations of the mean. Statistical difference to NA is shown: *p<0.05; **p<0.001.

FIG. 5: Representative photos of leaf 23 from untreated NA and LA plants and LA plants treated with polyamine biosynthesis inhibitors and/or plant growth regulator at harvest. D-arginine (5 mM) is an inhibitor of ADC; DFMO (difluoromethylornithine, 2 mM) is an inhibitor of ODC, ETH (Ethephon®, 0.5 mM) is a growth regulator.

FIG. 6: Treatment of LA plants with polyamine biosynthesis inhibitors and/or Ethephon®. Comparative total, free and conjugated polyamine in leaves (FIG. 6A) and roots (FIG. 6B) of untreated and treated LA plants at topping and harvest. Tobacco plants were grown in the greenhouse in the absence (NA and LA) or presence (LA) of polyamine biosynthesis inhibitors and/or Ethephon® (5 mM D-arginine, 2 mM DFMO, 2 mM DFMO/0.5 mM Ethephon® or 0.5 mM Ethephon® alone). D-arginine and DFMO were applied three times per week from before flowering to harvest for a period of 6 weeks, whereas Ethephon® treatment started after topping (2.5 weeks later) until harvest. Samples were collected 4 h after illumination from leaf 23 or roots of four biological replicates per genotype or treatment. The fold change between the mean polyamine content from untreated LA plants (gray bars), or plants treated with D-arginine (black bars), DFMO (white bars), DFMO/Ethephon® (horizontal lined bars), and Ethephon® (divot bars) are plotted. Error bars represent standard deviations of the mean (n=4). Statistical difference to the mean of LA/NA is indicated: *p<0.05. The red line represents polyamine content in NA.

BRIEF DESCRIPTION OF THE SEQUENCES

SEQ ID Nos: 1 to 11 set forth sequences of exemplary promoters for topping responsive root specific or preferred expression.

SEQ ID Nos: 11 to 21 set forth sequences of exemplary promoters for topping responsive leaf specific or preferred expression.

SEQ ID No: 22 sets forth a sequence of an exemplary DNA construct encoding a non-coding RNA suppressing an ornithine decarboxylase (ODC).

SEQ ID Nos: 23 to 28 set forth cDNA sequences of exemplary tobacco ODC genes.

SEQ ID Nos: 29 to 34 set forth amino acid sequences encoded by exemplary ODC genes.

SEQ ID Nos: 35 and 36 set forth two miRNA sequences targeting an ODC gene in accordance with the present disclosure.

Various sequences include “N” in nucleotide sequences or “X” in amino acid sequences. “N” can be any nucleotide, e.g., A, T, G, C, or a deletion or insertion of one or more nucleotides. In some instant, a string of “N” are shown. The number of “N” does not necessarily correlate with the actual number of undetermined nucleotides at that position. The actual nucleotide sequences can be longer or shorter than the shown segment of “N”. Similarly, “X” can be any amino acid residue or a deletion or insertion of one or more amino acids. Again, the number of “X” does not necessarily correlate with the actual number of undetermined amino acids at that position. The actual amino acid sequences can be longer or shorter than the shown segment of “X”. Notwithstanding the use of A, T, G, C (compared to A, U, G, C) in describing any SEQ ID in the sequence listing, that SEQ ID can also refer to a RNA sequence, depending on the context in which the SEQ ID is mentioned.

DETAILED DESCRIPTION

Unless defined otherwise, technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. One skilled in the art will recognize many methods can be used in the practice of the present disclosure. Indeed, the present disclosure is in no way limited to the methods and materials described. For purposes of the present disclosure, the following terms are defined below.

Any references cited herein, including, e.g., all patents and publications are incorporated by reference in their entirety.

As used herein, the singular form “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise. For example, the term “a compound” or “at least one compound” may include a plurality of compounds, including mixtures thereof.

The term “about” is used herein to mean approximately, roughly, around, or in the region of. When the term “about” is used in conjunction with a numerical range, it modifies that range by extending the boundaries above and below the numerical values set forth. To avoid any doubt, used herein, terms or phrases such as “about”, “at least”, “at least about”, “at most”, “less than”, “greater than”, “within” or alike, when followed by a series of list of numbers of percentages, such terms or phrases are deemed to modify each and every number of percentage in the series or list.

As used herein, a tobacco plant can be from any plant from the Nicotiana genus including, but not limited to Nicotiana tabacum, Nicotiana amplexicaulis PI 271989; Nicotiana benthamiana PI 555478; Nicotiana bigelovii PI 555485; Nicotiana debneyi; Nicotiana excelsior PI 224063; Nicotiana glutinosa PI 555507; Nicotiana goodspeedii PI 241012; Nicotiana gossei PI 230953; Nicotiana hesperis PI 271991; Nicotiana knightiana PI 555527; Nicotiana maritima PI 555535; Nicotiana megalosiphon PI 555536; Nicotiana nudicaulis PI 555540; Nicotiana paniculata PI 555545; Nicotiana plumbaginifolia PI 555548; Nicotiana repanda PI 555552; Nicotiana rustica; Nicotiana suaveolens PI 230960; Nicotiana sylvestris PI 555569; Nicotiana tomentosa PI 266379; Nicotiana tomentosiformis; and Nicotiana trigonophylla PI 555572.

In an aspect, the present disclosure provides tobacco plants, or part thereof, comprising an inducible promoter operably linked to a transcribable DNA sequence encoding a non-coding RNA for suppression of an ornithine decarboxylase (ODC) gene. In one aspect, tobacco plants comprise a mutation or a transgene conferring a reduced level of nicotine. In an aspect, tobacco plants are low-alkaloid tobacco plants. In one aspect, tobacco plants of the present disclosure comprise a nic1 mutation, a nic2 mutation, or both. In an aspect, tobacco plants comprise nicotine at a level below 1%, below 2%, below 5%, below 8%, below 10%, below 12%, below 15%, below 20%, below 25%, below 30%, below 40%, below 50%, below 60%, below 70%, or below 80% of the nicotine level of a control plant when grown in similar growth conditions, where the control plant shares an essentially identical genetic background with the tobacco plant except a low-nicotine conferring mutation or transgene. In another aspect, tobacco plants comprise nicotine or total alkaloids at a level below 1%, below 2%, below 5%, below 8%, below 10%, below 12%, below 15%, below 20%, below 25%, below 30%, below 40%, below 50%, below 60%, below 70%, or below 80% of the nicotine or total alkaloids level of the control plant when grown in similar growth conditions. In another aspect, tobacco plants comprise a total alkaloid level selected from the group consisting of less than 3%, less than 2.75%, less than 2.5%, less than 2.25%, less than 2.0%, less than 1.75%, less than 1.5%, less than 1.25%, less than 1%, less than 0.9%, less than 0.8%, less than 0.7%, less than 0.6%, less than 0.5%, less than 0.4%, less than 0.3%, less than 0.2%, less than 0.1%, and less than 0.05% of the nicotine level of a control plant when grown in similar growth conditions, where the control plant shares an essentially identical genetic background with the tobacco plant except a low-nicotine conferring mutation or transgene. In another aspect, tobacco plants comprise a nicotine or total alkaloid level selected from the group consisting of less than 3%, less than 2.75%, less than 2.5%, less than 2.25%, less than 2.0%, less than 1.75%, less than 1.5%, less than 1.25%, less than 1%, less than 0.9%, less than 0.8%, less than 0.7%, less than 0.6%, less than 0.5%, less than 0.4%, less than 0.3%, less than 0.2%, less than 0.1%, and less than 0.05% of the nicotine or total alkaloids level of the control plant when grown in similar growth conditions.

In an aspect, tobacco plants comprise a transgene or mutation directly suppressing the expression or activity of one or more, two or more, three or more, four or more, five or more, six or more, seven or more, eight or more, nine or more, ten or more, eleven or more, twelve or more, thirteen or more, fourteen or more, fifteen or more, sixteen or more, seventeen or more, eighteen or more, nineteen or more, twenty or more, or all twenty-one genes or loci encoding a protein selected from the group consisting of aspartate oxidase, agmatine deiminase (AIC), arginase, diamine oxidase, arginine decarboxylase (ADC), methylputrescine oxidase (MPO), NADH dehydrogenase, ornithine decarboxylase (ODC), phosphoribosylanthranilate isomerase (PRAI), putrescine N-methyltransferase (PMT), quinolate phosphoribosyl transferase (QPT), S-adenosyl-methionine synthetase (SAMS), A622, NBB1, BBL, MYC2, nic1, nic2, ethylene response factor (ERF) transcription factor, nicotine uptake permease (NUP), and MATE transporter. See Dewey and Xie, Molecular genetics of alkaloid biosynthesis in Nicotiana tabacum, Phytochemistry 94 (2013) 10-27.

In an aspect, tobacco plants further comprise one or more mutations in one or more, two or more, three or more, four or more, five or more, six or more, seven or more, eight or more, nine or more, or all ten genes selected from the group consisting of ERF32, ERF34, ERF39, ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168. In one aspect, tobacco plants further comprise one or more mutations in ERF189, ERF115, or both. In an aspect, tobacco plants further comprise one or more transgenes targeting and suppressing a gene encoding one or more, two or more, three or more, four or more, five or more, six or more, seven or more, eight or more, nine or more, or all ten proteins selected from the group consisting of ERF32, ERF34, ERF39, ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168.

In an aspect, tobacco plants are capable of producing leaves, when cured, having a USDA grade index value selected from the group consisting of 55 or more, 60 or more, 65 or more, 70 or more, 75 or more, 80 or more, 85 or more, 90 or more, and 95 or more. In another aspect, tobacco plants are capable of producing leaves, when cured, having a USDA grade index value comparable to that of a control plant when grown and cured in similar conditions, where the control plant shares an essentially identical genetic background with the tobacco plant except a low-nicotine conferring mutation or transgene. In a further aspect, tobacco plants are capable of producing leaves, when cured, having a USDA grade index value of at least about 65%, at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, or at least about 98% of the USDA grade index value of a control plant when grown in similar conditions, where the control plant shares an essentially identical genetic background with the tobacco plant except a low-nicotine conferring mutation or transgene. In a further aspect, tobacco plants are capable of producing leaves, when cured, having a USDA grade index value of between 65% and 130%, between 70% and 130%, between 75% and 130%, between 80% and 130%, between 85% and 130%, between 90% and 130%, between 95% and 130%, between 100% and 130%, between 105% and 130%, between 110% and 130%, between 115% and 130%, or between 120% and 130% of the USDA grade index value of the control plant. In a further aspect, tobacco plants are capable of producing leaves, when cured, having a USDA grade index value of between 70% and 125%, between 75% and 120%, between 80% and 115%, between 85% and 110%, or between 90% and 100% of the USDA grade index value of the control plant.

In another aspect, tobacco plants are capable of producing leaves, when cured, having a USDA grade index value selected from the group consisting of 55 or more, 60 or more, 65 or more, 70 or more, 75 or more, 80 or more, 85 or more, 90 or more, and 95 or more. In another aspect, tobacco plants are capable of producing leaves, when cured, having a USDA grade index value selected from the group consisting of between 50 and 95, between 55 and 95, between 60 and 95, between 65 and 95, between 70 and 95, between 75 and 95, between 80 and 95, between 85 and 95, between 90 and 95, between 55 and 90, between 60 and 85, between 65 and 80, between 70 and 75, between 50 and 55, between 55 and 60, between 60 and 65, between 65 and 70, between 70 and 75, between 75 and 80, between 80 and 85, between 85 and 90, and between 90 and 95. In a further aspect, tobacco plants are capable of producing leaves, when cured, having a USDA grade index value of at least about 65%, at least about 70%, at least about 75%, at least about 80%, at least about 85%, at least about 90%, at least about 95%, or at least about 98% of the USDA grade index value of the control plant. In a further aspect, tobacco plants are capable of producing leaves, when cured, having a USDA grade index value of between 65% and 130%, between 70% and 130%, between 75% and 130%, between 80% and 130%, between 85% and 130%, between 90% and 130%, between 95% and 130%, between 100% and 130%, between 105% and 130%, between 110% and 130%, between 115% and 130%, or between 120% and 130% of the USDA grade index value of the control plant. In a further aspect, tobacco plants are capable of producing leaves, when cured, having a USDA grade index value of between 70% and 125%, between 75% and 120%, between 80% and 115%, between 85% and 110%, or between 90% and 100% of the USDA grade index value of the control plant.

In an aspect, the present disclosure also provides a tobacco variety, cultivar, or line comprising a mutation selected from the group consisting of a nic1 mutation, a nic2 mutation, and a combination thereof, where the tobacco variety, cultivar, or line has a leaf grade comparable to the leaf grade of a control tobacco variety, cultivar, or line when grown in similar growth conditions, where the control tobacco variety shares an essentially identical genetic background with the tobacco variety, cultivar, or line except the mutation.

In an aspect, the present disclosure further provides non-transgenic tobacco plants, or part thereof, comprising a nicotine or total alkaloid level selected from the group consisting of less than 3%, less than 2.75%, less than 2.5%, less than 2.25%, less than 2.0%, less than 1.75%, less than 1.5%, less than 1.25%, less than 1%, less than 0.9%, less than 0.8%, less than 0.7%, less than 0.6%, less than 0.5%, less than 0.4%, less than 0.3%, less than 0.2%, less than 0.1%, and less than 0.05%, where the tobacco plants are capable of producing leaves, when cured, having a USDA grade index value of 50 or more 55 or more, 60 or more, 65 or more, 70 or more, 75 or more, 80 or more, 85 or more, 90 or more, and 95 or more. In another aspect, such non-transgenic tobacco plants comprise a nicotine level of less than 2.0% and are capable of producing leaves, when cured, having a USDA grade index value of 70 or more. In a further aspect, such non-transgenic tobacco plants comprise a nicotine level of less than 1.0% and are capable of producing leaves, when cured, having a USDA grade index value of 70 or more.

In an aspect, the present disclosure also provides a tobacco plant, or part thereof, comprising a non-transgenic mutation, where the non-transgenic mutation reduces the nicotine or total alkaloid level of the tobacco plant to below 1%, below 2%, below 5%, below 8%, below 10%, below 12%, below 15%, below 20%, below 25%, below 30%, below 40%, below 50%, below 60%, below 70%, or below 80% of the nicotine level of a control plant when grown in similar growth conditions, where the tobacco plant is capable of producing leaves, when cured, having a USDA grade index value comparable to the USDA grade index value of the control plant, and where the control plant shares an essentially identical genetic background with the tobacco plant except the non-transgenic mutation.

In an aspect, the present disclosure provides tobacco plants, or part thereof, comprising a mutation in a gene or locus, where the mutation is absent from LA Burley 21. In an aspect, tobacco plants provided herein comprise a shorter chromosomal introgression at a locus of interest compared to LA Burley 21. In another aspect, tobacco plants provided herein comprise no deletion of a complete gene or a complete genic coding sequence in the locus of interest. In an aspect, tobacco plants provided herein are homozygous at the locus of interest. In another aspect, tobacco plants provided herein are heterozygous at the locus of interest. In an aspect, tobacco plants provided herein comprise a mutation selected from the group consisting of a point mutation, a deletion, an insertion, a duplication, and an inversion at the gene or locus of interest. In an aspect, mutations in the tobacco plants provided herein are introduced by an approach selected from the group consisting of random mutagenesis and targeted mutagenesis. In another aspect, mutations in the tobacco plants provided herein are introduced by a targeted mutagenesis approach selected from the group consisting of meganuclease, zinc finger nuclease, TALEN, and CRISPR.

As used herein, a mutation refers to an inheritable genetic modification introduced into a gene to alter the expression or activity of a product encoded by the gene. Such a modification can be in any sequence region of a gene, for example, in a promoter, 5′ UTR, exon, intron, 3′ UTR, or terminator region. In an aspect, a mutation reduces, inhibits, or eliminates the expression or activity of a gene product. In another aspect, a mutation increases, elevates, strengthens, or augments the expression or activity of a gene product. In an aspect, mutations are not natural polymorphisms that exist in a particular tobacco variety or cultivar. As used herein, a “mutant allele” refers to an allele from a locus where the allele comprises a mutation. As used herein, “mutagenic” refers to generating a mutation without involving a transgene or with no mutation-related transgene remaining in an eventual mutant. In an aspect, mutagenic is cisgenic. In another aspect, mutagenic is via gene or genome editing. In a further aspect, mutagenic is via random mutagenesis, for example, chemical (e.g., EMS) or physical (r-irradiation) mutagenesis.

In an aspect, tobacco plants provided herein comprise one or more mutations within one or more genes comprising a coding sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, at least 98%, at least 99%, or 100% identity to a sequence selected from the group consisting of SEQ ID NOs: 23 to 28, and fragments thereof. In an aspect, one or more mutations reduce the expression or activity of one or more genes comprising a coding sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, at least 98%, at least 99%, or 100% identity to a sequence selected from the group consisting of SEQ ID NOs: 23 to 28, and fragments thereof.

In an aspect, tobacco plants provided herein comprise one or more mutations within one or more genes encoding a polypeptide having at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, at least 98%, at least 99%, or 100% identity to a sequence selected from the group consisting of SEQ ID NOs: 29 to 34, and fragments thereof. In an aspect, one or more mutations reduce the expression or activity of one or more genes encoding a polypeptide having at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, at least 98%, at least 99%, or 100% identity to a sequence selected from the group consisting of SEQ ID NOs: 29 to 34, and fragments thereof.

LA Burley 21 (also referenced as LA BU21) is a low total alkaloid tobacco line produced by incorporation of a low alkaloid gene(s) from a Cuban cigar variety into Burley 21 through several backcrosses (Legg et al. 1970). It has approximately 0.2% total alkaloids (dry weight) compared to the about 3.5% (dry weight) of its parent, Burley 21. LA BU21 has a leaf grade well below commercially acceptable standards. LA BU21 also exhibits other unfavorable leaf phenotypes characterized by lower yields, delayed ripening and senescence, higher susceptibility to insect herbivory, and poor end-product quality after curing (Chaplin and Weeks, 1976; Legg et al. 1970; Chaplin and Burk 1983). LA BU21 leaves further exhibit traits such as higher polyamine content, higher chlorophyll content and more mesophyll cells per unit leaf area.

In plants, polyamines are reportedly involved in developmental, physiological and metabolic processes such as cell growth and division, stress tolerance, vascular differentiation, lignin polymerization, pathogen defense, senescence and ripening (Fariduddin Q, Varshney P, Yusuf M, Ahmad A (2013) Polyamines: potent modulators of plant responses to stress. J. Plant Interac. 8: 1-16; Kusano T, Suzuki H (2015). Polyamines a universal molecular nexus for growth, survival and specialized metabolism. Tokyo: Springer.). Several studies have linked polyamines to the regulation of plant cell senescence (Sobieszczuk-Nowicka, E., Kubala, S., Zmienko, A., Malecka, A., Legocka, J. 2016. From accumulation to degradation: Reprograming polyamine metabolism facilitates dark-induced senescence in Barley leaf cells. Front. Plant Sci. doi: 10.3389/fpls.2015.01198). In fruit and vegetative tissues, polyamines act as anti-senescence and anti-ripening regulators that prevent the decay of chloroplast photosystem complexes and changes in cell wall/membrane composition (Lester G E (2000). Polyamines and their cellular anti-senescence properties in honey dew musk melon fruit. Plant Sci. 160: 105-112; Mattoo A K, Handa A K (2008). Higher polyamines restore and enhance metabolic memory in ripening fruit. Plant Sci. 174: 386-393; Serafini-Fracassini D, Di Sandro A, Del Duca S (2010). Spermine delays leaf senescence in Lactuca sativa and prevents the decay of chloroplast photosystems. Plant Physiol. Biochem. 48: 602-611). Higher levels of polyamines increase the longevity of tomato vines (Mehta R A, Cassol T, Li N, Ali N, Handa A K, Mattoo A K (2002). Engineered polyamine accumulation in tomato enhances phytonutrient content, juice quality and vine life. Nat. Biotechnol. 20: 613-618), and delayed ripening and leaf senescence was observed in transgenic tomato plants overexpressing a yeast spermidine synthase (Nambeesan S, Datsenka T, Ferruzzi M G, Malladi A, Mattoo A K, Handa A K (2010). Overexpression of yeast spermidine synthase impacts ripening, senescence and decay symptoms in tomato. Plant J. 63: 836-847). Polyamines may act directly by stabilizing cell walls or through crosstalk with phytohormones such as ethylene, abscisic acid, cytokinins and gibberellins (Kussano and Suzuki 2015).

In most plants, putrescine can be synthesized either directly from ornithine by ornithine decarboxylase (ODC) or from arginine via three enzymatic steps, initiated by arginine decarboxylase (ADC) (Michael A J, Furze J M, Rhodes M J, Burtin D (1996). Molecular cloning and functional identification of a plant ornithine decarboxylase cDNA. Biochem. J. 314: 241-248; Piotrowski M, Janowitz T, Kneifel H (2003). Plant C—N hydrolases and the identification of a plant N-carbamoylputrescine amidohydrolase involved in polyamine biosynthesis J. Biol. Chem. 278: 1708-1712; Illingworth C, Mayer M J, Elliot K, Hanfrey C, Walton N J, Michael A J (2003). The diverse bacterial origins of the Arabidopsis polyamine biosynthetic pathway FEBS Letters 549: 26-30). Previous studies have stated that the ADC route to putrescine has only a minor effect on the alkaloid profile of tobacco whereas the ODC pathway plays the major role in nicotine biosynthesis (Chintapakorn Y, Hamill J D (2007). Antisense-mediated reduction in ADC activity causes minor alterations in the alkaloid profile of cultured hairy root and regenerated transgenic plants of Nicotiana tabacum. Phytochem. 68: 2465-2479; DeBoer K D, Dalton H L, Edward F J, Hamill J D (2011). RNAi-mediated down-regulation of ornithine decarboxylase (ODC) leads to reduced nicotine and increased anatabine levels in transgenic Nicotiana tabacum L. Phytochem. 72: 344-355; DeBoer K D, Dalton H L, Edward F J, Ryan S M, Hamill J D (2013). RNAi-mediated down-regulation of ornithine decarboxylase (ODC) impedes wound-stress stimulation of anabasine synthesis in Nicotiana glauca. Phytochem. 86: 21-28; Dalton H L, Blomstedt C K, Neale A D, Gleadow R, DeBoer K D, Hamill J D (2016). Effects of down-regulating ornithine decarboxylase upon putrescine-associated metabolism and growth in Nicotiana tabacum L. J. Exp. Bot. 67: 3367-3381). Putrescine is converted to spermidine and then spermine by the successive addition of aminopropyl groups derived from decarboxylated S-adenosylmethionine (SAM), in reactions catalyzed by the enzymes spermidine synthase and spermine synthase, respectively. SAM is also a substrate for the biosynthesis of ethylene (Tiburcio A F, Altabella T, Bitrián M, Alcazar R (2014). The roles of polyamines during the lifespan of plants: from development to stress. Planta 240: 1-18), which regulates senescence and fruit ripening (Fluhr R, Mattoo A K (1996). Ethylene—biosynthesis and perception. Crit. Rev. Plant Sci. 15:479-523). The polyamine and ethylene biosynthesis pathways compete for the common precursor SAM but have opposing developmental effects, particularly during the developmental switch from vegetative growth to ripening/senescence (Nambeesan S, Handa A K, Mattoo A K (2008). Polyamines and regulation of ripening and senescence. In: Paliyath G, Murr D P, Handa A K, Lurie S (eds) Postharvest biology and technology of fruits, vegetables and flowers. Willey-Blackwell Publ, Ames. pp 319-340, Harpaz-Saad S, Yoon G M, Mattoo A K, Kieber J J (2012). The formation of ACC and competition between polyamines and ethylene for SAM. Annu. Plant Reviews. 44: 53-81, Gupta A, Pal R K, Rajam M V (2013). Delayed ripening and improved fruit processing quality in tomato by RNAi-mediated silencing of three homologs of 1-aminopropane-1-carboxylate synthase gene. J. Plant Physiol. 170: 987-995). Polyamine levels decrease and ethylene levels increase during the onset of fruit ripening in tomato (Saftner R A, Baldi B G (1990). Polyamine levels and tomato fruit development: possible interaction with ethylene. Plant Physiol. 92: 547-550; Morilla A, Garcia J M, Albi M A (1996). Free polyamine contents and decarboxylase activities during tomato development and ripening. J. Agri. Food Chem. 44: 2608-2611) and avocado (Kushad M M, Yelenosky G, Knight R (1988). Interrelationship of polyamine and ethylene biosynthesis during avocado fruit development and ripening. Plant Physiol. 87:463-467), which reflects the mutually antagonistic effect of ethylene on polyamine biosynthesis and vice versa (Harpaz-Saad et al. 2012; Anwar R, Mattoo A, Handa A (2015). Polyamine interactions with plant hormones: crosstalk at several levels in Kusano T, Suzuki H (eds). Polyamines a Universal Molecular Nexus for Growth, Survival and Specialized Metabolism. Tokyo: Springer. pp 267-303). However, transgenic tomato plants expressing yeast S-adenosylmethionine decarboxylase (SAMDC) under the control of the ripening-specific E8 promoter produced higher levels of ethylene and polyamines simultaneously during fruit ripening, indicating the absence of any competition for SAM in this system (Mehta R A, Cassol T, Li N, Ali N, Handa A K, Mattoo A K (2002). Engineered polyamine accumulation in tomato enhances phytonutrient content, juice quality and vine life. Nat. Biotechnol. 20: 613-618).

Without being bound to any scientific theory, the suppression of nicotine biosynthesis in LA tobacco plants can affect crosstalk between the nicotine, polyamine and ethylene pathways, resulting in the accumulation of putrescine. This would in turn increase metabolic flux towards the higher polyamines spermidine and spermine while inhibiting ethylene biosynthesis, causing a dramatic effect on leaf ripening and senescence.

In an aspect, the present disclosure provides tobacco plants, or part thereof, comprising a low nicotine or low alkaloid-conferring mutation or transgene and capable of producing a leaf comprising a comparable level of one or more polyamines relative to a comparable leaf of a control plant not comprising the same mutation or transgene. In one aspect, a comparable level of one or more polyamines is within 20%, 17.5%, 15%, 12.5%, 10%, 7.5%, 5%, 2.5%, or 1% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene. In an aspect, a comparable level of one or more polyamines is between 0.5% and 1%, between 1% and 2%, between 2% and 3%, between 3% and 4%, between 4% and 5%, between 5% and 6%, between 6% and 7%, between 7% and 8%, between 8% and 9%, between 9% and 10%, between 11% and 12%, between 12% and 13%, between 13% and 14%, between 14% and 15%, between 15% and 16%, between 16% and 17%, between 17% and 18%, between 18% and 19%, or between 19% and 20% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene. In a further aspect, a comparable level of one or more polyamines is between 0.5% and 5%, between 5% and 10%, or between 10% and 20% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene.

In an aspect, the present disclosure provides tobacco plants, or part thereof, comprising a low nicotine or low alkaloid-conferring mutation or transgene and capable of producing a leaf comprising a comparable chlorophyll level relative to a comparable leaf of a control plant not comprising the same mutation or transgene. In one aspect, a comparable chlorophyll level is within 20%, 17.5%, 15%, 12.5%, 10%, 7.5%, 5%, 2.5%, or 1% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene. In an aspect, a comparable chlorophyll level is between 0.5% and 1%, between 1% and 2%, between 2% and 3%, between 3% and 4%, between 4% and 5%, between 5% and 6%, between 6% and 7%, between 7% and 8%, between 8% and 9%, between 9% and 10%, between 11% and 12%, between 12% and 13%, between 13% and 14%, between 14% and 15%, between 15% and 16%, between 16% and 17%, between 17% and 18%, between 18% and 19%, or between 19% and 20% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene. In a further aspect, a comparable chlorophyll level is between 0.5% and 5%, between 5% and 10%, or between 10% and 20% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene.

In an aspect, the present disclosure provides tobacco plants, or part thereof, comprising a low nicotine or low alkaloid-conferring mutation or transgene and capable of producing a leaf comprising a comparable number of mesophyll cell per unit of leaf area relative to a comparable leaf of a control plant not comprising the same mutation or transgene. In one aspect, a comparable number of mesophyll cell per unit of leaf area is within 20%, 17.5%, 15%, 12.5%, 10%, 7.5%, 5%, 2.5%, or 1% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene. In an aspect, a comparable number of mesophyll cell per unit of leaf area is between 0.5% and 1%, between 1% and 2%, between 2% and 3%, between 3% and 4%, between 4% and 5%, between 5% and 6%, between 6% and 7%, between 7% and 8%, between 8% and 9%, between 9% and 10%, between 11% and 12%, between 12% and 13%, between 13% and 14%, between 14% and 15%, between 15% and 16%, between 16% and 17%, between 17% and 18%, between 18% and 19%, or between 19% and 20% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene. In a further aspect, a comparable number of mesophyll cell per unit of leaf area is between 0.5% and 5%, between 5% and 10%, or between 10% and 20% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene.

In an aspect, the present disclosure provides tobacco plants, or part thereof, comprising a low nicotine or low alkaloid-conferring mutation or transgene and capable of producing a leaf comprising a comparable epidermal cell size relative to a comparable leaf of a control plant not comprising the same mutation or transgene. In one aspect, a comparable epidermal cell size is within 20%, 17.5%, 15%, 12.5%, 10%, 7.5%, 5%, 2.5%, or 1% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene. In an aspect, a comparable epidermal cell size is between 0.5% and 1%, between 1% and 2%, between 2% and 3%, between 3% and 4%, between 4% and 5%, between 5% and 6%, between 6% and 7%, between 7% and 8%, between 8% and 9%, between 9% and 10%, between 11% and 12%, between 12% and 13%, between 13% and 14%, between 14% and 15%, between 15% and 16%, between 16% and 17%, between 17% and 18%, between 18% and 19%, or between 19% and 20% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene. In a further aspect, a comparable epidermal cell size is between 0.5% and 5%, between 5% and 10%, or between 10% and 20% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene.

In an aspect, the present disclosure provides tobacco plants, or part thereof, comprising a low nicotine or low alkaloid-conferring mutation or transgene and capable of producing a leaf comprising a comparable leaf yield relative to a comparable leaf of a control plant not comprising the same mutation or transgene. In one aspect, a comparable leaf yield is within 20%, 17.5%, 15%, 12.5%, 10%, 7.5%, 5%, 2.5%, or 1% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene. In an aspect, a comparable leaf yield is between 0.5% and 1%, between 1% and 2%, between 2% and 3%, between 3% and 4%, between 4% and 5%, between 5% and 6%, between 6% and 7%, between 7% and 8%, between 8% and 9%, between 9% and 10%, between 11% and 12%, between 12% and 13%, between 13% and 14%, between 14% and 15%, between 15% and 16%, between 16% and 17%, between 17% and 18%, between 18% and 19%, or between 19% and 20% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene. In a further aspect, a comparable leaf yield is between 0.5% and 5%, between 5% and 10%, or between 10% and 20% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene.

In an aspect, the present disclosure provides tobacco plants, or part thereof, comprising a low nicotine or low alkaloid-conferring mutation or transgene and exhibiting a comparable insect herbivory susceptibility relative to a comparable leaf of a control plant not comprising the same mutation or transgene. In one aspect, a comparable insect herbivory susceptibility is within 20%, 17.5%, 15%, 12.5%, 10%, 7.5%, 5%, 2.5%, or 1% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene. In an aspect, a comparable insect herbivory susceptibility is between 0.5% and 1%, between 1% and 2%, between 2% and 3%, between 3% and 4%, between 4% and 5%, between 5% and 6%, between 6% and 7%, between 7% and 8%, between 8% and 9%, between 9% and 10%, between 11% and 12%, between 12% and 13%, between 13% and 14%, between 14% and 15%, between 15% and 16%, between 16% and 17%, between 17% and 18%, between 18% and 19%, or between 19% and 20% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene. In a further aspect, a comparable insect herbivory susceptibility is between 0.5% and 5%, between 5% and 10%, or between 10% and 20% of the level in a comparable leaf of a control plant not comprising the same mutation or transgene.

Insect herbivory susceptibility level can be assayed by methods known in the art, for example, in an insect feeding assay. In short, a quarter inch layer of 0.7% agar in water is added to a 100 mm Petri dish and allowed to solidify. Leaf discs are cut from the petri dish lid, placed in the plates and pushed gently into the agar. Leaf discs are taken from plants at the 4-5 leaf stage. Discs were taken from lamina only to exclude major midribs. A single disc is taken from each of the four largest leaves of the plant generating 4 replicates per plant. Four plants are sampled for a total of 16 biological replicates test line. A single budworm at the second instar stage is added to the leaf and allowed to feed for 48 hours at ambient temperature. After 48 hours the budworm larvae are weighed and final larval weights are recorded.

Unless specified otherwise, measurements of alkaloid, polyamine, or nicotine levels (or another leaf chemistry or property characterization) or leaf grade index values mentioned herein for a tobacco plant, variety, cultivar, or line can refer to average measurements, including, for example, depending on the context, an average of multiple leaves of a single plant or an average measurement from a population of tobacco plants from a single variety, cultivar, or line. In an aspect, the nicotine, alkaloid, or polyamine level (or another leaf chemistry or property characterization) of a tobacco plant is measured after topping in a pooled leaf sample collected from leaf number 3, 4, and 5 after topping. In another aspect, the nicotine, alkaloid, or polyamine level (or another leaf chemistry or property characterization) of a tobacco plant is measured after topping in a leaf having the highest level of nicotine, alkaloid, or polyamine (or another leaf chemistry or property characterization). In an aspect, the nicotine, alkaloid, or polyamine level of a tobacco plant is measured after topping in leaf number 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, or 30. In another aspect, the nicotine, alkaloid, or polyamine level (or another leaf chemistry or property characterization) of a tobacco plant is measured after topping in a pool of two or more leaves with consecutive leaf numbers selected from the group consisting of leaf number 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, and 30. In another aspect, the nicotine, alkaloid, or polyamine level (or another leaf chemistry or property characterization) of a tobacco plant is measured after topping in a leaf with a leaf number selected from the group consisting of between 1 and 5, between 6 and 10, between 11 and 15, between 16 and 20, between 21 and 25, and between 26 and 30. In another aspect, the nicotine, alkaloid, or polyamine level (or another leaf chemistry or property characterization) of a tobacco plant is measured after topping in a pool of two or more leaves with leaf numbers selected from the group consisting of between 1 and 5, between 6 and 10, between 11 and 15, between 16 and 20, between 21 and 25, and between 26 and 30. In another aspect, the nicotine, alkaloid, or polyamine level (or another leaf chemistry or property characterization) of a tobacco plant is measured after topping in a pool of three or more leaves with leaf numbers selected from the group consisting of between 1 and 5, between 6 and 10, between 11 and 15, between 16 and 20, between 21 and 25, and between 26 and 30.

Alkaloid levels can be assayed by methods known in the art, for example by quantification based on gas-liquid chromatography, high performance liquid chromatography, radio-immunoassays, and enzyme-linked immunosorbent assays.

As used herein, leaf numbering is based on the leaf position on a tobacco stalk with leaf number 1 being the oldest leaf (at the base) after topping and the highest leaf number assigned to the youngest leaf (at the tip).

A population of tobacco plants or a collection of tobacco leaves for determining an average measurement (e.g., alkaloid or nicotine level or leaf grading) can be of any size, for example, 5, 10, 15, 20, 25, 30, 35, 40, or 50. Industry-accepted standard protocols are followed for determining average measurements or grad index values.

As used herein, “topping” refers to the removal of the stalk apex, including the SAM, flowers, and up to several adjacent leaves, when a tobacco plant is near vegetative maturity and around the start of reproductive growth. Typically, tobacco plants are topped in the button stage (soon after the flower begins to appear). For example, greenhouse or field-grown tobacco plants can be topped when 50% of the plants have at least one open flower. Topping a tobacco plant results in the loss of apical dominance and also induce increased alkaloid production.

Typically, the nicotine, alkaloid, or polyamine level (or another leaf chemistry or property characterization) of a tobacco plant is measured about 2 weeks after topping. Other time points can also be used. In an aspect, the nicotine, alkaloid, or polyamine level (or another leaf chemistry or property characterization) of a tobacco plant is measured about 1, 2, 3, 4, or 5 weeks after topping. In another aspect, the nicotine, alkaloid, or polyamine level (or another leaf chemistry or property characterization) of a tobacco plant is measured about 3, 5, 7, 10, 12, 14, 17, 19, or 21 days after topping.

As used herein, “similar growth conditions” refer to similar environmental conditions and/or agronomic practices for growing and making meaningful comparisons between two or more plant genotypes so that neither environmental conditions nor agronomic practices would contribute to or explain any difference observed between the two or more plant genotypes. Environmental conditions include, for example, light, temperature, water (humidity), and nutrition (e.g., nitrogen and phosphorus). Agronomic practices include, for example, seeding, clipping, undercutting, transplanting, topping, and suckering. See Chapters 4B and 4C of Tobacco, Production, Chemistry and Technology, Davis & Nielsen, eds., Blackwell Publishing, Oxford (1999), pp 70-103.

As used herein, “comparable leaves” refer to leaves having similar size, shape, age, and/or stalk position.

In an aspect, the present disclosure provides tobacco plants, or part thereof, comprising an inducible promoter operably linked to a transcribable DNA sequence encoding a non-coding RNA for suppression of an ornithine decarboxylase (ODC) gene. In one aspect, an inducible promoter is a topping-inducible promoter. In an aspect, an inducible promoter is also a tissue-specific or tissue-preferred promoter. In one aspect, a tissue-specific or tissue-preferred promoter is specific or preferred for one or more tissues or organs selected from the group consisting of shoot, root, leaf, stem, flower, sucker, root tip, mesophyll cells, epidermal cells, and vasculature. In a further aspect, a topping inducible promoter comprises a promoter sequence from a tobacco nicotine demethylase gene, for example, CYP82E4, CYP82E5, or CYP82E10.

Various types of promoters can be used here, which are classified according to a variety of criteria relating to the pattern of expression of a coding sequence or gene (including a transgene) operably linked to the promoter, such as constitutive, developmental, tissue-specific, tissue-preferred, inducible, etc. Promoters that initiate transcription in all or most tissues of the plant are referred to as “constitutive” promoters. Promoters that initiate transcription during certain periods or stages of development are referred to as “developmental” promoters. Promoters whose expression is enhanced in certain tissues of the plant relative to other plant tissues are referred to as “tissue-enhanced” or “tissue-preferred” promoters. Thus, a “tissue-preferred” promoter causes relatively higher or preferential expression in a specific tissue(s) of the plant, but with lower levels of expression in other tissue(s) of the plant. Promoters that express within a specific tissue(s) of the plant, with little or no expression in other plant tissues, are referred to as “tissue-specific” promoters. A promoter that expresses in a certain cell type of the plant is referred to as a “cell type specific” promoter. An “inducible” promoter is a promoter that initiates transcription in response to an environmental stimulus such as cold, drought, heat or light, or other stimuli, such as wounding or chemical application. A promoter may also be classified in terms of its origin, such as being heterologous, homologous, chimeric, synthetic, etc. A “heterologous” promoter is a promoter sequence having a different origin relative to its associated transcribable sequence, coding sequence, or gene (or transgene), and/or not naturally occurring in the plant species to be transformed. The term “heterologous” more broadly includes a combination of two or more DNA molecules or sequences when such a combination is not normally found in nature. For example, two or more DNA molecules or sequences would be heterologous with respect to each other if they are normally found in different genomes or at different loci in the same genome, or if they are not identically combined in nature.

In an aspect, an inducible promoter provides root specific or preferred expression. In one aspect, a root specific or preferred inducible promoter comprises a sequence selected from the group consisting of SEQ ID Nos: 1-11 and a functional fragment thereof. Table 1 provides a comparison of estimated leaf versus root specific expression level driven by SEQ ID Nos: 1-11.

In an aspect, an inducible promoter provides leaf specific or preferred expression. In one aspect, a leaf specific or preferred inducible promoter comprises a sequence selected from the group consisting of SEQ ID Nos: 12-21 and a functional fragment thereof. Table 2 provides a comparison of estimated leaf versus root specific expression level driven by SEQ ID Nos: 12-21.

TABLE 1 Exemplary inducible promoters for topping-responsive root specific or preferred expression Root Leaf 3 days after 3 days 4 wks 3 days topping Gene Layby Before after after Layby Flowering after (nitrogen SEQ Id Id stage Topping Topping Topping Senescence stage time Topping deficient) SEQ ID g78655 0.0 0.0 0.0 0.0 1.1 1.7 1.0 146.0 251.0 NO: 1 SEQ ID g72021 0.0 0.0 1.7 0.7 3.3 4.3 6.9 501.7 468.7 NO: 2 SEQ ID g78252 3.2 0.0 0.0 1.5 0.0 0.9 3.0 77.3 167.8 NO: 3 SEQ ID g65720 0.0 0.0 1.7 0.0 3.8 0.9 0.0 33.5 6.0 NO: 4 SEQ ID g74108 3.2 3.3 0.0 0.7 3.6 2.6 4.0 81.6 142.9 NO: 5 SEQ ID g47466 12.7 31.6 0.0 69.6 43.2 0.0 1.0 22.3 10.6 NO: 6 SEQ ID g102868 3.2 3.3 0.0 0.0 0.2 0.9 1.0 36.9 23.4 NO: 7 SEQ ID g23057 0.0 0.0 1.7 0.0 8.7 2.6 13.8 247.4 287.2 NO: 8 SEQ ID g34684 12.7 41.6 43.0 67.4 104.0 21.5 105.7 1947.5 2177.0 NO: 9 SEQ ID g105948 15.8 3.3 0.0 8.8 11.9 16.3 14.8 78.2 768.7 NO: 10 SEQ ID g81261 145.6 68.3 72.3 90.2 73.1 44.7 115.6 2206.9 2112.0 NO: 11

TABLE 2 Exemplary inducible promoters for topping-responsive leaf specific or preferred expression 1 day 3 days 1 week 2 weeks 3 weeks Before after after after after after Harvest SEQ Id Gene Topping Topping Topping Topping Topping Topping time SEQ ID g2237 20 170 344 305 706 432 1049 NO: 12 SEQ ID g31142 41 61 177 480 167 1530 892 NO: 13 SEQ ID g75488 41 9 306 534 47 2297 138 NO: 14 SEQ ID g94193 119 128 846 1650 377 4337 1546 NO: 15 SEQ ID g34756 15 86 113 73 22 183 611 NO: 16 SEQ ID g104299 48 110 166 576 138 1521 994 NO: 17 SEQ ID g44810 21 19 99 80 492 140 521 NO: 18 SEQ ID g71671 15 26 58 210 40 348 299 NO: 19 SEQ ID g29427 34 34 395 314 474 591 339 NO: 20 SEQ ID g49024 29 28 229 230 280 382 595 NO: 21

In an aspect, an inducible promoter is a heterologous to the operably linked transcribable DNA sequence. In one aspect, a transcribable DNA sequence encodes a non-coding RNA selected from the group consisting of microRNA (miRNA), anti-sense RNA, small interfering RNA (siRNA), a trans-acting siRNA (ta-siRNA), and hairpin RNA (hpRNA). In an aspect, a non-coding RNA comprises a nucleotide sequence having at least 99.9%, at least 99.5%, at least 99%, at least 98%, at least 97%, at least 96%, at least 95%, at least 94%, at least 93%, at least 92%, at least 91%, at least 90%, at least 85%, at least 80%, or at least 75% identity to a sequence selected from the group consisting of SEQ ID Nos: 35 and 36, and any portions thereof. In one aspect, a non-coding RNA is provided in an ODC RNAi construct comprising a nucleotide sequence having at least 99%, at least 97%, at least 95%, at least 90%, at least 85%, at least 80%, or at least 75% identity to SEQ ID No: 22.

“Alkaloids” are complex, nitrogen-containing compounds that naturally occur in plants, and have pharmacological effects in humans and animals. “Nicotine” is the primary natural alkaloid in commercialized cigarette tobacco and accounts for about 90 percent of the alkaloid content in Nicotiana tabacum. Other major alkaloids in tobacco include cotinine, nornicotine, myosmine, nicotyrine, anabasine and anatabine. Minor tobacco alkaloids include nicotine-n-oxide, N-methyl anatabine, N-methyl anabasine, pseudooxynicotine, 2,3 dipyridyl and others.

In an aspect, tobacco plants provided herein comprise a lower level of total alkaloid or an individual alkaloid compared to a control tobacco plant without a nic1 mutation and/or a nic2 mutation when grown in similar growth conditions. In another aspect, tobacco plants provided herein comprise a lower level of one or more alkaloids selected from the group consisting of cotinine, nornicotine, myosmine, nicotyrine, anabasine and anatabine, compared to a control tobacco plant when grown in similar growth conditions. In an aspect, a lower alkaloid or nicotine level refers to an alkaloid or nicotine level of below 1%, below 2%, below 5%, below 8%, below 10%, below 12%, below 15%, below 20%, below 25%, below 30%, below 40%, below 50%, below 60%, below 70%, or below 80% of the alkaloid or nicotine level of a control tobacco plant. In another aspect, a lower alkaloid or nicotine level refers to an alkaloid or nicotine level of about between 0.5% and 1%, between 1% and 2%, between 2% and 3%, between 3% and 4%, between 4% and 5%, between 5% and 6%, between 6% and 7%, between 7% and 8%, between 8% and 9%, between 9% and 10%, between 11% and 12%, between 12% and 13%, between 13% and 14%, between 14% and 15%, between 15% and 16%, between 16% and 17%, between 17% and 18%, between 18% and 19%, between 19% and 20%, between 21% and 22%, between 22% and 23%, between 23% and 24%, between 24% and 25%, between 25% and 26%, between 26% and 27%, between 27% and 28%, between 28% and 29%, or between 29% and 30% of the alkaloid or nicotine level of a control tobacco plant. In a further aspect, a lower alkaloid or nicotine level refers to an alkaloid or nicotine level of about between 0.5% and 5%, between 5% and 10%, between 10% and 20%, between 20% and 30% of the alkaloid or nicotine level of a control tobacco plant.

Alkaloid levels can be assayed by methods known in the art, for example by quantification based on gas-liquid chromatography, high performance liquid chromatography, radio-immunoassays, and enzyme-linked immunosorbent assays. For example, nicotinic alkaloid levels can be measured by a GC-FID method based on CORESTA Recommended Method No. 7, 1987 and ISO Standards (ISO TC 126N 394 E. See also Hibi et al., Plant Physiology 100: 826-35 (1992) for a method using gas-liquid chromatography equipped with a capillary column and an FID detector. Unless specified otherwise, all alkaloid levels described here are measured using a method in accordance with CORESTA Method No 62, Determination of Nicotine in Tobacco and Tobacco Products by Gas Chromatographic Analysis, February 2005, and those defined in the Centers for Disease Control and Prevention's Protocol for Analysis of Nicotine, Total Moisture and pH in Smokeless Tobacco Products, as published in the Federal Register Vol. 64, No. 55 Mar. 23, 1999 (and as amended in Vol. 74, No. 4, Jan. 7, 2009).

Alternatively, tobacco total alkaloids can be measured using a segmented-flow colorimetric method developed for analysis of tobacco samples as adapted by Skalar Instrument Co (West Chester, Pa.) and described by Collins et al., Tobacco Science 13:79-81 (1969). In short, samples of tobacco are dried, ground, and extracted prior to analysis of total alkaloids and reducing sugars. The method then employs an acetic acid/methanol/water extraction and charcoal for decolorization. Determination of total alkaloids was based on the reaction of cyanogen chloride with nicotine alkaloids in the presence of an aromatic amine to form a colored complex which is measured at 460 nm. Unless specified otherwise, total alkaloid levels or nicotine levels shown herein are on a dry weight basis (e.g., percent total alkaloid or percent nicotine).

In an aspect, tobacco plants provided herein comprise a lower level of nicotine compared to a control tobacco plant without a nic1 mutation and/or a nic2 mutation when grown in similar growth conditions. In an aspect, a lower nicotine level refers to an average nicotine level of below 1%, below 2%, below 5%, below 8%, below 10%, below 12%, below 15%, below 20%, below 25%, below 30%, below 40%, below 50%, below 60%, below 70%, or below 80% of the average nicotine level of a control tobacco plant. In another aspect, a lower nicotine level refers to an average nicotine level of about between 0.5% and 1%, between 1% and 2%, between 2% and 3%, between 3% and 4%, between 4% and 5%, between 5% and 6%, between 6% and 7%, between 7% and 8%, between 8% and 9%, between 9% and 10%, between 11% and 12%, between 12% and 13%, between 13% and 14%, between 14% and 15%, between 15% and 16%, between 16% and 17%, between 17% and 18%, between 18% and 19%, between 19% and 20%, between 21% and 22%, between 22% and 23%, between 23% and 24%, between 24% and 25%, between 25% and 26%, between 26% and 27%, between 27% and 28%, between 28% and 29%, or between 29% and 30% of the average nicotine level of a control tobacco plant. In a further aspect, a lower nicotine level refers to an average nicotine level of about between 0.5% and 5%, between 5% and 10%, between 10% and 20%, between 20% and 30% of the average nicotine level of a control tobacco plant.

In an aspect, tobacco plants provided herein comprise an average nicotine or total alkaloid level selected from the group consisting of about 0.01%, 0.02%, 0.05%, 0.75%, 0.1%, 0.15%, 0.2%, 0.3%, 0.35%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, 3.1%, 3.2%, 3.3%, 3.4%, 3.5%, 3.6%, 3.7%, 3.8%, 3.9%, 4%, 5%, 6%, 7%, 8%, and 9% on a dry weight basis. In another aspect, tobacco plants provided herein comprise an average nicotine or total alkaloid level selected from the group consisting of about between 0.01% and 0.02%, between 0.02% and 0.05%, between 0.05% and 0.75%, between 0.75% and 0.1%, between 0.1% and 0.15%, between 0.15% and 0.2%, between 0.2% and 0.3%, between 0.3% and 0.35%, between 0.35% and 0.4%, between 0.4% and 0.5%, between 0.5% and 0.6%, between 0.6% and 0.7%, between 0.7% and 0.8%, between 0.8% and 0.9%, between 0.9% and 1%, between 1% and 1.1%, between 1.1% and 1.2%, between 1.2% and 1.3%, between 1.3% and 1.4%, between 1.4% and 1.5%, between 1.5% and 1.6%, between 1.6% and 1.7%, between 1.7% and 1.8%, between 1.8% and 1.9%, between 1.9% and 2%, between 2% and 2.1%, between 2.1% and 2.2%, between 2.2% and 2.3%, between 2.3% and 2.4%, between 2.4% and 2.5%, between 2.5% and 2.6%, between 2.6% and 2.7%, between 2.7% and 2.8%, between 2.8% and 2.9%, between 2.9% and 3%, between 3% and 3.1%, between 3.1% and 3.2%, between 3.2% and 3.3%, between 3.3% and 3.4%, between 3.4% and 3.5%, and between 3.5% and 3.6% on a dry weight basis. In a further aspect, tobacco plants provided herein comprise an average nicotine or total alkaloid level selected from the group consisting of about between 0.01% and 0.1%, between 0.02% and 0.2%, between 0.03% and 0.3%, between 0.04% and 0.4%, between 0.05% and 0.5%, between 0.75% and 1%, between 0.1% and 1.5%, between 0.15% and 2%, between 0.2% and 3%, and between 0.3% and 3.5% on a dry weight basis.

The present disclosure also provides tobacco plants having altered nicotine levels without negative impacts over other tobacco traits, e.g., leaf grade index value. In an aspect, a low-nicotine or nicotine-free tobacco variety provides cured tobacco of commercially acceptable grade. Tobacco grades are evaluated based on factors including, but not limited to, the leaf stalk position, leaf size, leaf color, leaf uniformity and integrity, ripeness, texture, elasticity, sheen (related to the intensity and the depth of coloration of the leaf as well as the shine), hygroscopicity (the faculty of the tobacco leaves to absorb and to retain the ambient moisture), and green nuance or cast. Leaf grade can be determined, for example, using an Official Standard Grade published by the Agricultural Marketing Service of the US Department of Agriculture (7 U.S.C. § 511). See, e.g., Official Standard Grades for Burley Tobacco (U.S. Type 31 and Foreign Type 93), effective Nov. 5, 1990 (55 F.R. 40645); Official Standard Grades for Flue-Cured Tobacco (U.S. Types 11, 12, 13, 14 and Foreign Type 92), effective Mar. 27, 1989 (54 F.R. 7925); Official Standard Grades for Pennsylvania Seedleaf Tobacco (U.S. Type 41), effective Jan. 8, 1965 (29 F.R. 16854); Official Standard Grades for Ohio Cigar-Leaf Tobacco (U.S. Types 42, 43, and 44), effective Dec. 8, 1963 (28 F.R. 11719 and 28 F.R. 11926); Official Standard Grades for Wisconsin Cigar-Binder Tobacco (U.S. Types 54 and 55), effective Nov. 20, 1969 (34 F.R. 17061); Official Standard Grades for Wisconsin Cigar-Binder Tobacco (U.S. Types 54 and 55), effective Nov. 20, 1969 (34 F.R. 17061); Official Standard Grades for Georgia and Florida Shade-Grown Cigar-Wrapper Tobacco (U.S. Type 62), Effective April 1971. A USDA grade index value can be determined according to an industry accepted grade index. See, e.g., Bowman et al, Tobacco Science, 32:39-40(1988); Legacy Tobacco Document Library (Bates Document #523267826-523267833, Jul. 1, 1988, Memorandum on the Proposed Burley Tobacco Grade Index); and Miller et al., 1990, Tobacco Intern., 192:55-57 (all foregoing references are incorporated by inference in their entirety). In an aspect, a USDA grade index is a 0-100 numerical representation of federal grade received and is a weighted average of all stalk positions. A higher grade index indicates higher quality. Alternatively, leaf grade can be determined via hyper-spectral imaging. See e.g., WO 2011/027315 (published on Mar. 10, 2011, and incorporated by inference in its entirety). A comparable leaf grade index indicates a leaf grade index that does not vary more than 30% above or below an appropriate control or comparator when comparing leaves from similar stalk positions. In an aspect, a comparable leaf grade index does not vary more than 25%, 20%, 15%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1% above or below an appropriate control or comparator when comparing leaves from similar stalk positions.

In an aspect, tobacco plants provided herein comprise a similar level of one or more tobacco aroma compounds selected from the group consisting of 3-methylvaleric acid, valeric acid, isovaleric acid, a labdenoid, a cembrenoid, a sugar ester, and a reducing sugar, compared to control tobacco plants when grown in similar growth conditions. In another aspect, tobacco plants provided herein comprise a nic1 mutation, a nic2 mutation, or a combination thereof having no impact over the level of one or more tobacco aroma compounds selected from the group consisting of 3-methylvaleric acid, valeric acid, isovaleric acid, a labdenoid, a cembrenoid, a sugar ester, and a reducing sugar.

As used herein, tobacco aroma compounds are compounds associated with the flavor and aroma of tobacco smoke. These compounds include, but are not limited to, 3-methylvaleric acid, valeric acid, isovaleric acid, cembrenoid and labdenoid diterpenes, and sugar esters. Concentrations of tobacco aroma compounds can be measured by any known metabolite profiling methods in the art including, without limitation, gas chromatography mass spectrometry (GC-MS), Nuclear Magnetic Resonance Spectroscopy, liquid chromatography-linked mass spectrometry. See The Handbook of Plant Metabolomics, edited by Weckwerth and Kahl, (Wiley-Blackwell) (May 28, 2013).

As used herein, “reducing sugar(s)” are any sugar (monosaccharide or polysaccharide) that has a free or potentially free aldehdye or ketone group. Glucose and fructose act as nicotine buffers in cigarette smoke by reducing smoke pH and effectively reducing the amount of “free” unprotonated nicotine. Reducing sugars balances smoke flavor, for example, by modifying the sensory impact of nicotine and other tobacco alkaloids. An inverse relationship between sugar content and alkaloid content has been reported across tobacco varieties, within the same variety, and within the same plant line caused by planting conditions. Reducing sugar levels can be measured using a segmented-flow colorimetric method developed for analysis of tobacco samples as adapted by Skalar Instrument Co (West Chester, Pa.) and described by Davis, Tobacco Science 20:139-144 (1976). For example, a sample is dialyzed against a sodium carbonate solution. Copper neocuproin is added to the sample and the solution is heated. The copper neocuproin chelate is reduced in the presence of sugars resulting in a colored complex which is measured at 460 nm.

In an aspect, tobacco plants provided herein comprise one or more non-naturally existing mutant alleles at nic1 and/or nic2 locus which reduce or eliminate one or more gene activity from nic1 and/or nic2 locus. In an aspect, these mutant alleles result in lower nicotine levels. Mutant nic1 and/or nic2 alleles can be introduced by any method known in the art including random or targeted mutagenesis approaches.

Such mutagenesis methods include, without limitation, treatment of seeds with ethyl methylsulfate (EMS) (Hildering and Verkerk, In, The use of induced mutations in plant breeding. Pergamon press, pp 317-320, 1965) or UV-irradiation, X-rays, and fast neutron irradiation (see, for example, Verkerk, Neth. J. Agric. Sci. 19:197-203, 1971; and Poehlman, Breeding Field Crops, Van Nostrand Reinhold, N.Y. (3.sup.rd ed), 1987), transposon tagging (Fedoroff et al., 1984; U.S. Pat. Nos. 4,732,856 and 5,013,658), as well as T-DNA insertion methodologies (Hoekema et al., 1983; U.S. Pat. No. 5,149,645). EMS-induced mutagenesis consists of chemically inducing random point mutations over the length of the genome. Fast neutron mutagenesis consists of exposing seeds to neutron bombardment which causes large deletions through double stranded DNA breakage. Transposon tagging comprises inserting a transposon within an endogenous gene to reduce or eliminate expression of the gene. The types of mutations that may be present in a tobacco gene include, for example, point mutations, deletions, insertions, duplications, and inversions. Such mutations desirably are present in the coding region of a tobacco gene; however mutations in the promoter region, and intron, or an untranslated region of a tobacco gene may also be desirable.

In addition, a fast and automatable method for screening for chemically induced mutations, TILLING (Targeting Induced Local Lesions In Genomes), using denaturing HPLC or selective endonuclease digestion of selected PCR products is also applicable to the present disclosure. See, McCallum et al. (2000) Nat. Biotechnol. 18:455-457. Mutations that impact gene expression or that interfere with the function of genes can be determined using methods that are well known in the art. Insertional mutations in gene exons usually result in null-mutants. Mutations in conserved residues can be particularly effective in inhibiting the function of a protein. In an aspect, tobacco plants comprise a nonsense (e.g., stop codon) mutation in one or more NCG genes described in U.S. Provisional Application Nos. 62/616,959 and 62/625,878, both of which are incorporated by reference in their entirety.

In an aspect, the present disclosure also provides tobacco lines with altered nicotine levels while maintaining commercially acceptable leaf quality. These lines can be produced by introducing mutations into one or more genes at nic1 and/or nic2 locus via precise genome engineering technologies, for example, Transcription activator-like effector nucleases (TALENs), meganuclease, zinc finger nuclease, and a clustered regularly-interspaced short palindromic repeats (CRISPR)/Cas9 system, a CRISPR/Cpf1 system, a CRISPR/Csm1 system, and a combination thereof (see, for example, U.S. Patent Application publication 2017/0233756). See, e.g., Gaj et al., Trends in Biotechnology, 31(7):397-405 (2013).

The screening and selection of mutagenized tobacco plants can be through any methodologies known to those having ordinary skill in the art. Examples of screening and selection methodologies include, but are not limited to, Southern analysis, PCR amplification for detection of a polynucleotide, Northern blots, RNase protection, primer-extension, RT-PCR amplification for detecting RNA transcripts, Sanger sequencing, Next Generation sequencing technologies (e.g., Illumina, PacBio, Ion Torrent, 454), enzymatic assays for detecting enzyme or ribozyme activity of polypeptides and polynucleotides, and protein gel electrophoresis, Western blots, immunoprecipitation, and enzyme-linked immunoassays to detect polypeptides. Other techniques such as in situ hybridization, enzyme staining, and immunostaining also can be used to detect the presence or expression of polypeptides and/or polynucleotides. Methods for performing all of the referenced techniques are known.

In an aspect, a tobacco plant or plant genome provided herein is mutated or edited by a nuclease selected from the group consisting of a meganuclease, a zinc-finger nuclease (ZFN), a transcription activator-like effector nuclease (TALEN), a CRISPR/Cas9 nuclease, a CRISPR/Cpf1 nuclease, or a CRISPR/Csm1 nuclease.

In an aspect, tobacco plants provided herein comprising a nic1 mutation, a nic2 mutation, or both, further comprises a transgene or mutation providing an early-senescence trait. In one aspect, a mutation providing an early-senescence trait is yellow burley1 (-yb1). In an aspect, a mutation providing an early-senescence trait is yellow burley2 (-yb2). In one aspect, a mutation providing an early-senescence trait is pale yellow (PY).

In an aspect, a tobacco plant, or part thereof, comprises relative to a control tobacco plant: a first genome modification providing a lower level of nicotine or total alkaloid, and a second genome modification providing a comparable level of one or more traits selected from the group consisting of total leaf polyamine level, total root polyamine level, total leaf chlorophyll level, mesophyll cell number per leaf area unit, and leaf epidermal cell size; and where the control plant does not have both the first and the second genome modifications. In one aspect, a tobacco plant, or part thereof, comprises relative to a control tobacco plant: a first genome modification providing a lower level of nicotine or total alkaloid, and a second genome modification providing a comparable level of total leaf polyamine level, where the control plant does not have both the first and the second genome modifications. In an aspect, a tobacco plant, or part thereof, comprises relative to a control tobacco plant: a first genome modification providing a lower level of nicotine or total alkaloid, and a second genome modification providing a comparable level of total root polyamine level, where the control plant does not have both the first and the second genome modifications. In one aspect, a tobacco plant, or part thereof, comprises relative to a control tobacco plant: a first genome modification providing a lower level of nicotine or total alkaloid, and a second genome modification providing a comparable level of total leaf chlorophyll level, where the control plant does not have both the first and the second genome modifications. In an aspect, a tobacco plant, or part thereof, comprises relative to a control tobacco plant: a first genome modification providing a lower level of nicotine or total alkaloid, and a second genome modification providing a comparable level of mesophyll cell number per leaf area unit, where the control plant does not have both the first and the second genome modifications. In one aspect, a tobacco plant, or part thereof, comprises relative to a control tobacco plant: a first genome modification providing a lower level of nicotine or total alkaloid, and a second genome modification providing a comparable level of leaf epidermal cell size, where the control plant does not have both the first and the second genome modifications.

In an aspect, a first genome modification, a second genome modification, or both comprise a transgene, a mutation, or both. In one aspect, a genome modification, a second genome modification, or both comprise a transgene. In an aspect, a first genome modification, a second genome modification, or both comprise a mutation. In one aspect, a first genome modification, a second genome modification, or both are not transgene-based. In an aspect, a first genome modification, a second genome modification, or both are not mutation-based.

In an aspect, tobacco plants provided herein comprise a first genome modification providing a lower level of nicotine compared to a control tobacco plant. In one aspect, tobacco plants provided herein comprise a first genome modification comprising a nic1 mutation, a nic2 mutation, or both. In an aspect, tobacco plants provided herein comprise a transgene targeting the Nic1 locus, a transgene targeting the Nic2 locus, or both.

In an aspect, tobacco plants provided herein comprise a first genome modification comprising a mutation in a gene or locus encoding a protein selected from the group consisting of aspartate oxidase, agmatine deiminase (AIC), arginase, diamine oxidase, arginine decarboxylase (ADC), methylputrescine oxidase (MPO), NADH dehydrogenase, ornithine decarboxylase (ODC), phosphoribosylanthranilate isomerase (PRAI), putrescine N-methyltransferase (PMT), quinolate phosphoribosyl transferase (QPT), and S-adenosyl-methionine synthetase (SAMS), A622, NBB1, BBL, MYC2, Nic1, Nic2, ethylene response factor (ERF) transcription factor, nicotine uptake permease (NUP), and MATE transporter. In one aspect, tobacco plants provided herein comprise a first genome modification comprises a transgene targeting and suppressing a gene or locus encoding a protein selected from the group consisting of aspartate oxidase, agmatine deiminase (AIC), arginase, diamine oxidase, arginine decarboxylase (ADC), methylputrescine oxidase (MPO), NADH dehydrogenase, ornithine decarboxylase (ODC), phosphoribosylanthranilate isomerase (PRAI), putrescine N-methyltransferase (PMT), quinolate phosphoribosyl transferase (QPT), and S-adenosyl-methionine synthetase (SAMS), A622, NBB1, BBL, MYC2, Nic1, Nic2, ethylene response factor (ERF) transcription factor, nicotine uptake permease (NUP), and MATE transporter.

In an aspect, tobacco plants provided herein comprise a first genome modification comprising a mutation in a gene or locus encoding a protein selected from the group consisting of ERF32, ERF34, ERF39, ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168. In one aspect, tobacco plants provided herein comprise a first genome modification comprises a transgene targeting and suppressing a gene or locus encoding a protein selected from the group consisting of ERF32, ERF34, ERF39, ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168.

In an aspect, a tobacco plant or plant genome provided is mutated or edited to have one or more mutations in one or more NCG genes selected from the group consisting of NCG1 to NCG35, where the one or more mutations reduce or eliminate the activity or expression of the one or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have one or more mutations in one or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG13, NCG15, NCG16, NCG17, NCG21, NCG22, NCG24, NCG26, NCG29, NCG30, and NCG35, where the one or more mutations reduce or eliminate the activity or expression of the one or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have one or more mutations in one or more NCG genes selected from the group consisting of NCG1 to NCG21, where the one or more mutations reduce or eliminate the activity or expression of the one or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have one or more mutations in one or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG15, NCG16, and NCG17, where the one or more mutations reduce or eliminate the activity or expression of the one or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have one or more mutations in one or more NCG genes selected from the group consisting of NCG2, NCG12, NCG15, NCG16, and NCG17, where the one or more mutations reduce or eliminate the activity or expression of the one or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have one or more mutations in one or more NCG genes selected from the group consisting of NCG12, NCG15, NCG16, and NCG17, where the one or more mutations reduce or eliminate the activity or expression of the one or more NCG genes. In another aspect, an edited or mutated tobacco plant having one or more, two or more, three or more, four or more, or five or more NCG mutations further comprises one or more mutations in one or more, two or more, three or more, four or more, or five or more ERF genes selected from the group consisting of ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168, where the one or more mutations reduce or eliminate the activity or expression of the one or more ERF genes.

In an aspect, a tobacco plant or plant genome provided is mutated or edited to have two or more mutations in two or more NCG genes selected from the group consisting of NCG1 to NCG35, where the two or more mutations reduce or eliminate the activity or expression of the two or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have two or more mutations in two or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG13, NCG15, NCG16, NCG17, NCG21, NCG22, NCG24, NCG26, NCG29, NCG30, and NCG35, where the two or more mutations reduce or eliminate the activity or expression of the two or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have two or more mutations in two or more NCG genes selected from the group consisting of NCG1 to NCG21, where the two or more mutations reduce or eliminate the activity or expression of the one or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have two or more mutations in two or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG15, NCG16, and NCG17, where the two or more mutations reduce or eliminate the activity or expression of the one or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have two or more mutations in two or more NCG genes selected from the group consisting of NCG2, NCG12, NCG15, NCG16, and NCG17, where the two or more mutations reduce or eliminate the activity or expression of the two or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have two or more mutations in two or more NCG genes selected from the group consisting of NCG12, NCG15, NCG16, and NCG17, where the two or more mutations reduce or eliminate the activity or expression of the two or more NCG genes. In another aspect, an edited or mutated tobacco plant having two or more NCG mutations further comprises two or more mutations in two or more ERF genes selected from the group consisting of ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168, where the two or more mutations reduce or eliminate the activity or expression of the two or more ERF genes.

In an aspect, a tobacco plant or plant genome provided is mutated or edited to have three or more mutations in three or more NCG genes selected from the group consisting of NCG1 to NCG35, where the three or more mutations reduce or eliminate the activity or expression of the three or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have three or more mutations in three or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG13, NCG15, NCG16, NCG17, NCG21, NCG22, NCG24, NCG26, NCG29, NCG30, and NCG35, where the three or more mutations reduce or eliminate the activity or expression of the three or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have three or more mutations in three or more NCG genes selected from the group consisting of NCG1 to NCG21, where the three or more mutations reduce or eliminate the activity or expression of the three or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have three or more mutations in three or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG15, NCG16, and NCG17, where the three or more mutations reduce or eliminate the activity or expression of the three or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have three or more mutations in three or more NCG genes selected from the group consisting of NCG2, NCG12, NCG15, NCG16, and NCG17, where the three or more mutations reduce or eliminate the activity or expression of the three or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have three or more mutations in three or more NCG genes selected from the group consisting of NCG12, NCG15, NCG16, and NCG17, where the three or more mutations reduce or eliminate the activity or expression of the three or more NCG genes. In another aspect, an edited or mutated tobacco plant having three or more NCG mutations further comprises three or more mutations in three or more ERF genes selected from the group consisting of ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168, where the three or more mutations reduce or eliminate the activity or expression of the three or more ERF genes.

In an aspect, a tobacco plant or plant genome provided is mutated or edited to have four or more mutations in four or more NCG genes selected from the group consisting of NCG1 to NCG35, where the four or more mutations reduce or eliminate the activity or expression of the four or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have four or more mutations in four or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG13, NCG15, NCG16, NCG17, NCG21, NCG22, NCG24, NCG26, NCG29, NCG30, and NCG35, where the four or more mutations reduce or eliminate the activity or expression of the four or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have four or more mutations in four or more NCG genes selected from the group consisting of NCG1 to NCG21, where the four or more mutations reduce or eliminate the activity or expression of the four or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have four or more mutations in four or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG15, NCG16, and NCG17, where the four or more mutations reduce or eliminate the activity or expression of the four or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have four or more mutations in four or more NCG genes selected from the group consisting of NCG2, NCG12, NCG15, NCG16, and NCG17, where the four or more mutations reduce or eliminate the activity or expression of the four or more NCG genes. In an aspect, a tobacco plant or plant genome provided is mutated or edited to have four or more mutations in four or more NCG genes selected from the group consisting of NCG12, NCG15, NCG16, and NCG17, where the four or more mutations reduce or eliminate the activity or expression of the four or more NCG genes. In another aspect, an edited or mutated tobacco plant having four or more NCG mutations further comprises four or more mutations in four or more ERF genes selected from the group consisting of ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168, where the four or more mutations reduce or eliminate the activity or expression of the four or more ERF genes.

In an aspect, a tobacco plant or plant genome provided comprises one or more transgenes targeting one or more NCG genes selected from the group consisting of NCG1 to NCG35, where the one or more transgenes reduce or eliminate the activity or expression of the one or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises one or more transgenes targeting one or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG13, NCG15, NCG16, NCG17, NCG21, NCG22, NCG24, NCG26, NCG29, NCG30, and NCG35, where the one or more transgenes reduce or eliminate the activity or expression of the one or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises one or more transgenes targeting one or more NCG genes selected from the group consisting of NCG1 to NCG21, where the one or more transgenes reduce or eliminate the activity or expression of the one or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises one or more transgenes targeting one or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG15, NCG16, and NCG17, where the one or more transgenes reduce or eliminate the activity or expression of the one or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises one or more transgenes targeting one or more NCG genes selected from the group consisting of NCG2, NCG12, NCG15, NCG16, and NCG17, where the one or more transgenes reduce or eliminate the activity or expression of the one or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises one or more transgenes targeting one or more NCG genes selected from the group consisting of NCG12, NCG15, NCG16, and NCG17, where the one or more transgenes reduce or eliminate the activity or expression of the one or more NCG genes. In another aspect, a tobacco plant having one or more, two or more, three or more, four or more, or five or more NCG-targeting transgenes further comprises one or more mutations in one or more, two or more, three or more, four or more, or five or more ERF genes selected from the group consisting of ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168, where the one or more mutations reduce or eliminate the activity or expression of the one or more ERF genes. In another aspect, a tobacco plant having one or more, two or more, three or more, four or more, or five or more NCG-targeting transgenes further comprises one or more transgenes targeting one or more, two or more, three or more, four or more, or five or more ERF genes selected from the group consisting of ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168, where the one or more ERF-targeting transgenes reduce or eliminate the activity or expression of the one or more ERF genes.

In an aspect, a tobacco plant or plant genome provided comprises two or more transgenes targeting two or more NCG genes selected from the group consisting of NCG1 to NCG35, where the two or more transgenes reduce or eliminate the activity or expression of the two or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises two or more transgenes targeting two or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG13, NCG15, NCG16, NCG17, NCG21, NCG22, NCG24, NCG26, NCG29, NCG30, and NCG35, where the two or more transgenes reduce or eliminate the activity or expression of the two or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises two or more transgenes targeting two or more NCG genes selected from the group consisting of NCG1 to NCG21, where the two or more transgenes reduce or eliminate the activity or expression of the one or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises two or more transgenes targeting two or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG15, NCG16, and NCG17, where the two or more transgenes reduce or eliminate the activity or expression of the one or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises two or more transgenes targeting two or more NCG genes selected from the group consisting of NCG2, NCG12, NCG15, NCG16, and NCG17, where the two or more transgenes reduce or eliminate the activity or expression of the two or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises two or more transgenes targeting two or more NCG genes selected from the group consisting of NCG12, NCG15, NCG16, and NCG17, where the two or more transgenes reduce or eliminate the activity or expression of the two or more NCG genes. In another aspect, a tobacco plant having two or more NCG-targeting transgenes further comprises two or more mutations in two or more ERF genes selected from the group consisting of ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168, where the two or more mutations reduce or eliminate the activity or expression of the two or more ERF genes. In another aspect, a tobacco plant having two or more NCG-targeting transgenes further comprises two or more transgenes targeting two or more ERF genes selected from the group consisting of ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168, where the two or more ERF-targeting transgenes reduce or eliminate the activity or expression of the two or more ERF genes.

In an aspect, a tobacco plant or plant genome provided comprises three or more transgenes targeting three or more NCG genes selected from the group consisting of NCG1 to NCG35, where the three or more transgenes reduce or eliminate the activity or expression of the three or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises three or more transgenes targeting three or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG13, NCG15, NCG16, NCG17, NCG21, NCG22, NCG24, NCG26, NCG29, NCG30, and NCG35, where the three or more transgenes reduce or eliminate the activity or expression of the three or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises three or more transgenes targeting three or more NCG genes selected from the group consisting of NCG1 to NCG21, where the three or more transgenes reduce or eliminate the activity or expression of the three or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises three or more transgenes targeting three or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG15, NCG16, and NCG17, where the three or more transgenes reduce or eliminate the activity or expression of the three or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises three or more transgenes targeting three or more NCG genes selected from the group consisting of NCG2, NCG12, NCG15, NCG16, and NCG17, where the three or more transgenes reduce or eliminate the activity or expression of the three or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises three or more transgenes targeting three or more NCG genes selected from the group consisting of NCG12, NCG15, NCG16, and NCG17, where the three or more transgenes reduce or eliminate the activity or expression of the three or more NCG genes. In another aspect, a tobacco plant having three or more NCG-targeting transgenes further comprises three or more mutations in three or more ERF genes selected from the group consisting of ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168, where the three or more mutations reduce or eliminate the activity or expression of the three or more ERF genes. In another aspect, a tobacco plant having three or more NCG-targeting transgenes further comprises three or more transgenes targeting three or more ERF genes selected from the group consisting of ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168, where the three or more ERF-targeting transgenes reduce or eliminate the activity or expression of the three or more ERF genes.

In an aspect, a tobacco plant or plant genome provided comprises four or more transgenes targeting four or more NCG genes selected from the group consisting of NCG1 to NCG35, where the four or more transgenes reduce or eliminate the activity or expression of the four or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises four or more transgenes targeting four or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG13, NCG15, NCG16, NCG17, NCG21, NCG22, NCG24, NCG26, NCG29, NCG30, and NCG35, where the four or more transgenes reduce or eliminate the activity or expression of the four or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises four or more transgenes targeting four or more NCG genes selected from the group consisting of NCG1 to NCG21, where the four or more transgenes reduce or eliminate the activity or expression of the four or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises four or more transgenes targeting four or more NCG genes selected from the group consisting of NCG1, NCG2, NCG11, NCG12, NCG15, NCG16, and NCG17, where the four or more transgenes reduce or eliminate the activity or expression of the four or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises four or more transgenes targeting four or more NCG genes selected from the group consisting of NCG2, NCG12, NCG15, NCG16, and NCG17, where the four or more transgenes reduce or eliminate the activity or expression of the four or more NCG genes. In an aspect, a tobacco plant or plant genome provided comprises four or more transgenes targeting four or more NCG genes selected from the group consisting of NCG12, NCG15, NCG16, and NCG17, where the four or more transgenes reduce or eliminate the activity or expression of the four or more NCG genes. In another aspect, a tobacco plant having four or more NCG-targeting transgenes further comprises four or more mutations in four or more ERF genes selected from the group consisting of ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168, where the four or more mutations reduce or eliminate the activity or expression of the four or more ERF genes. In another aspect, a tobacco plant having four or more NCG-targeting transgenes further comprises four or more transgenes targeting four or more ERF genes selected from the group consisting of ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168, where the four or more ERF-targeting transgenes reduce or eliminate the activity or expression of the four or more ERF genes.

In an aspect, tobacco plants provided herein comprise second genome modification comprising an inducible promoter operably linked to a transcribable DNA sequence encoding a non-coding RNA for suppression of an ornithine decarboxylase (ODC) gene. In another aspect, a tobacco plant is provided having suppressed MYB8 activity via either transgene suppression, mutagenesis, or targeted genome editing. For simplicity, every instance here mentioning ODC suppression (e.g., operably linked to any particular type of promoters) is equally applicable to MYB8 suppression.

In an aspect, tobacco plants provided herein comprise a reduced amount of total conjugated polyamines in leaves relative to the control tobacco plant. In one aspect, tobacco plants provided herein comprise a reduced amount of total conjugated polyamines in roots relative to the control tobacco plant. Used here, conjugated polyamines include, but are not limited to, soluble conjugated polyamines such as phenolamides containing a backbone consisting of a free polyamine (e.g., putrescine, spermine, and/or spermidine) conjugated with one or more phenylpropanoids such as ferulic, caffeic and courmaric acids. Conjugated polyamines also include, but are not limited to, insoluble conjugated polyamines incorporated into structural polymers such as lignin. In an aspect, tobacco plants provided herein comprise a reduced amount of total free polyamines (e.g., putrescine, spermine, and spermidine) in leaves relative to the control tobacco plant. In one aspect, tobacco plants provided herein comprise a reduced amount of total conjugated polyamines in roots relative to the control tobacco plant. In an aspect, tobacco plants provided herein comprise a reduced amount of total conjugated form of one or more polyamines selected from the group consisting of putrescine, spermidine and spermine in leaves relative to the control tobacco plant. In one aspect, tobacco plants provided herein comprise a reduced amount of total conjugated form of one or more polyamines selected from the group consisting of putrescine, spermidine and spermine in roots relative to the control tobacco plant. In an aspect, tobacco plants provided herein comprise a reduced amount of total free form of one or more polyamines selected from the group consisting of putrescine, spermidine and spermine in leaves relative to the control tobacco plant. In one aspect, tobacco plants provided herein comprise a reduced amount of total conjugated form of one or more polyamines selected from the group consisting of putrescine, spermidine and spermine in roots relative to the control tobacco plant.

In an aspect, a characteristic or a trait of a tobacco plant described here are measured at a time selected from the group consisting of immediately before flowering, at topping, 1 week-post-topping (WPT), 2 WPT, 3 WPT, 4 WPT, 5 WPT, 6 WPT, 7 WPT, 8 WPT, and at harvest. In one aspect, tobacco plants provided herein comprising a first and a second genome modification are capable of producing a leaf with a leaf grade comparable to that of a leaf from a control plant. In an aspect, tobacco plants provided herein comprising a first and a second genome modification have a total leaf yield comparable to a control plant.

As used herein, “editing” or “genome editing” refers to targeted mutagenesis of at least 1, at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, or at least 10 nucleotides of an endogenous plant genome nucleic acid sequence, or removal or replacement of an endogenous plant genome nucleic acid sequence. In an aspect, an edited nucleic acid sequence provided has at least 99.9%, at least 99.5%, at least 99%, at least 98%, at least 97%, at least 96%, at least 95%, at least 94%, at least 93%, at least 92%, at least 91%, at least 90%, at least 85%, at least 80%, or at least 75% sequence identity with an endogenous nucleic acid sequence. In an aspect, an edited nucleic acid sequence provided has at least 99.9%, at least 99.5%, at least 99%, at least 98%, at least 97%, at least 96%, at least 95%, at least 94%, at least 93%, at least 92%, at least 91%, at least 90%, at least 85%, at least 80%, or at least 75% sequence identity with SEQ ID Nos: 23-28, and fragments thereof. In another aspect, an edited nucleic acid sequence provided has at least 99.9%, at least 99.5%, at least 99%, at least 98%, at least 97%, at least 96%, at least 95%, at least 94%, at least 93%, at least 92%, at least 91%, at least 90%, at least 85%, at least 80%, or at least 75% sequence identity with a polynucleotide encoding a polypeptide selected from the group consisting of SEQ ID NOs:29-34.

Meganucleases, ZFNs, TALENs, CRISPR/Cas9, CRISPR/Csm1 and CRISPR/Cpf1 induce a double-strand DNA break at a target site of a genomic sequence that is then repaired by the natural processes of homologous recombination (HR) or non-homologous end-joining (NHEJ). Sequence modifications then occur at the cleaved sites, which can include deletions or insertions that result in gene disruption in the case of NHEJ, or integration of donor nucleic acid sequences by HR. In an aspect, a method provided comprises editing a plant genome with a nuclease provided to mutate at least 1, at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9, at least 10, or more than 10 nucleotides in the plant genome via HR with a donor polynucleotide. In an aspect, a mutation provided is caused by genome editing using a nuclease. In another aspect, a mutation provided is caused by non-homologous end-joining or homologous recombination.

In an aspect, a mutation provided here provides a dominant mutant that activates the expression or activity of a gene of interest, e.g., a gene selected from the group consisting of a biosynthetic enzyme, a regulatory transcription factor, a transporter, a catabolic enzyme, or a combination thereof, for one or more antioxidants.

Meganucleases, which are commonly identified in microbes, are unique enzymes with high activity and long recognition sequences (>14 bp) resulting in site-specific digestion of target DNA. Engineered versions of naturally occurring meganucleases typically have extended DNA recognition sequences (for example, 14 to 40 bp). The engineering of meganucleases can be more challenging than that of ZFNs and TALENs because the DNA recognition and cleavage functions of meganucleases are intertwined in a single domain. Specialized methods of mutagenesis and high-throughput screening have been used to create novel meganuclease variants that recognize unique sequences and possess improved nuclease activity.

ZFNs are synthetic proteins consisting of an engineered zinc finger DNA-binding domain fused to the cleavage domain of the FokI restriction endonuclease. ZFNs can be designed to cleave almost any long stretch of double-stranded DNA for modification of the zinc finger DNA-binding domain. ZFNs form dimers from monomers composed of a non-specific DNA cleavage domain of FokI endonuclease fused to a zinc finger array engineered to bind a target DNA sequence.

The DNA-binding domain of a ZFN is typically composed of 3-4 zinc-finger arrays. The amino acids at positions −1, +2, +3, and +6 relative to the start of the zinc finger ∞-helix, which contribute to site-specific binding to the target DNA, can be changed and customized to fit specific target sequences. The other amino acids form the consensus backbone to generate ZFNs with different sequence specificities. Rules for selecting target sequences for ZFNs are known in the art.

The FokI nuclease domain requires dimerization to cleave DNA and therefore two ZFNs with their C-terminal regions are needed to bind opposite DNA strands of the cleavage site (separated by 5-7 bp). The ZFN monomer can cute the target site if the two-ZF-binding sites are palindromic. The term ZFN, as used herein, is broad and includes a monomeric ZFN that can cleave double stranded DNA without assistance from another ZFN. The term ZFN is also used to refer to one or both members of a pair of ZFNs that are engineered to work together to cleave DNA at the same site.

Without being limited by any scientific theory, because the DNA-binding specificities of zinc finger domains can in principle be re-engineered using one of various methods, customized ZFNs can theoretically be constructed to target nearly any gene sequence. Publicly available methods for engineering zinc finger domains include Context-dependent Assembly (CoDA), Oligomerized Pool Engineering (OPEN), and Modular Assembly.

TALENs are artificial restriction enzymes generated by fusing the transcription activator-like effector (TALE) DNA binding domain to a FokI nuclease domain. When each member of a TALEN pair binds to the DNA sites flanking a target site, the FokI monomers dimerize and cause a double-stranded DNA break at the target site. The term TALEN, as used herein, is broad and includes a monomeric TALEN that can cleave double stranded DNA without assistance from another TALEN. The term TALEN is also used to refer to one or both members of a pair of TALENs that work together to cleave DNA at the same site.

Transcription activator-like effectors (TALEs) can be engineered to bind practically any DNA sequence. TALE proteins are DNA-binding domains derived from various plant bacterial pathogens of the genus Xanthomonas. The Xanthomonas pathogens secrete TALEs into the host plant cell during infection. The TALE moves to the nucleus, where it recognizes and binds to a specific DNA sequence in the promoter region of a specific DNA sequence in the promoter region of a specific gene in the host genome. TALE has a central DNA-binding domain composed of 13-28 repeat monomers of 33-34 amino acids. The amino acids of each monomer are highly conserved, except for hypervariable amino acid residues at positions 12 and 13. The two variable amino acids are called repeat-variable diresidues (RVDs). The amino acid pairs NI, NG, HD, and NN of RVDs preferentially recognize adenine, thymine, cytosine, and guanine/adenine, respectively, and modulation of RVDs can recognize consecutive DNA bases. This simple relationship between amino acid sequence and DNA recognition has allowed for the engineering of specific DNA binding domains by selecting a combination of repeat segments containing the appropriate RVDs.

Besides the wild-type FokI cleavage domain, variants of the FokI cleavage domain with mutations have been designed to improve cleavage specificity and cleavage activity. The FokI domain functions as a dimer, requiring two constructs with unique DNA binding domains for sites in the target genome with proper orientation and spacing. Both the number of amino acid residues between the TALEN DNA binding domain and the FokI cleavage domain and the number of bases between the two individual TALEN binding sites are parameters for achieving high levels of activity.

A relationship between amino acid sequence and DNA recognition of the TALE binding domain allows for designable proteins. Software programs such as DNA Works can be used to design TALE constructs. Other methods of designing TALE constructs are known to those of skill in the art. See Doyle et al, Nucleic Acids Research (2012) 40: W117-122.; Cermak et al., Nucleic Acids Research (2011). 39:e82; and tale-nt.cac.cornell.edu/about.

A CRISPR/Cas9 system, CRISPR/Csm1, or a CRISPR/Cpf1 system are alternatives to the FokI-based methods ZFN and TALEN. The CRISPR systems are based on RNA-guided engineered nucleases that use complementary base pairing to recognize DNA sequences at target sites.

CRISPR/Cas9, CRISPR/Csm1, and a CRISPR/Cpf1 systems are part of the adaptive immune system of bacteria and archaea, protecting them against invading nucleic acids such as viruses by cleaving the foreign DNA in a sequence-dependent manner. The immunity is acquired by the integration of short fragments of the invading DNA known as spacers between two adjacent repeats at the proximal end of a CRISPR locus. The CRISPR arrays, including the spacers, are transcribed during subsequent encounters with invasive DNA and are processed into small interfering CRISPR RNAs (crRNAs) approximately 40 nt in length, which combine with the trans-activating CRISPR RNA (tracrRNA) to activate and guide the Cas9 nuclease. This cleaves homologous double-stranded DNA sequences known as protospacers in the invading DNA. A prerequisite for cleavage is the presence of a conserved protospacer-adjacent motif (PAM) downstream of the target DNA, which usually has the sequence 5-NGG-3 but less frequently NAG. Specificity is provided by the so-called “seed sequence” approximately 12 bases upstream of the PAM, which must match between the RNA and target DNA. Cpf1 and Csm1 act in a similar manner to Cas9, but Cpf1 and Csm1 do not require a tracrRNA.

In still another aspect, a tobacco plant provided further comprises one or more mutations in one or more loci encoding a nicotine demethylase (e.g., CYP82E4, CYP82E5, CYP82E10) that confer reduced amounts of nornicotine (See U.S. Pat. Nos. 8,319,011; 8,124,851; 9,187,759; 9,228,194; 9,228,195; 9,247,706) compared to control plant lacking one or more mutations in one or more loci encoding a nicotine demethylase. In an aspect, a modified tobacco plant described further comprises reduced nicotine demethylase activity compared to a control plant when grown and cured under comparable conditions. In a further aspect, a tobacco plant provided further comprises one or more mutations or transgenes providing an elevated level of one or more antioxidants (See U.S. patent application Ser. No. 15/727,523 and PCT Application No. PCT/US2017/055618). In another aspect, a tobacco plant provided further comprises one or more mutations or transgenes providing a reduced level of one or more TSNAs (such as N′-nitrosonornicotine (NNN), 4-methylnitrosoamino-1-(3-pyridyl)-1-butanone (NNK), N′-nitrosoanatabine (NAT) N′-nitrosoanabasine (NAB)).

The present disclosure also provides compositions and methods for inhibiting the expression or function of one or more genes involved in polyamine biosynthesis or regulation thereof, in a plant, particularly plants of the Nicotiana genus, including tobacco plants of the various commercial varieties.

In an aspect, the present disclosure provides tobacco plants, or part thereof, comprising a heterologous expression cassette comprising an ODC inhibitory sequence. In another aspect, tobacco plants, or part thereof, comprise a heterologous expression cassette comprising an inhibitory sequence of a gene comprising a sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, at least 98%, at least 99%, or 100% identity to a sequence selected from the group consisting of SEQ ID NOs: 23-28, and fragments thereof, where the inhibitory sequence is operably linked to a promoter that is functional in a plant cell, and where the inhibitory sequence has at least 90% sequence identity to a fragment of at least 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, or 80 nucleotides of the sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, at least 98%, at least 99%, or 100% identity to a sequence selected from the group consisting of SEQ ID NOs: 23-28, and fragments thereof.

As used herein, the terms “inhibit,” “inhibition,” and “inhibiting” are defined as any method known in the art or described herein that decreases the expression or function of a gene product of interest (e.g., a target gene product). “Inhibition” can be in the context of a comparison between two plants, for example, a genetically altered plant versus a wild-type plant. Alternatively, inhibition of expression or function of a target gene product can be in the context of a comparison between plant cells, organelles, organs, tissues, or plant parts within the same plant or between different plants, and includes comparisons between developmental or temporal stages within the same plant or plant part or between plants or plant parts. “Inhibition” includes any relative decrement of function or production of a gene product of interest, up to and including complete elimination of function or production of that gene product. The term “inhibition” encompasses any method or composition that down-regulates translation and/or transcription of the target gene product or functional activity of the target gene product. In an aspect, the mRNA or protein level of one or more genes in a modified plant is less than 95%, less than 90%, less than 80%, less than 70%, less than 60%, less than 50%, less than 40%, less than 30%, less than 20%, less than 10%, less than 5%, less than 4%, less than 3%, less than 2%, or less than 1% of the mRNA or protein level of the same gene in a plant that is not a mutant or that has not been genetically modified to inhibit the expression of that gene.

The term “inhibitory sequence” encompasses any polynucleotide or polypeptide sequence capable of inhibiting the expression or function of a gene involved in nicotine biosynthesis regulation from Nic1b locus in a plant, such as full-length polynucleotide or polypeptide sequences, truncated polynucleotide or polypeptide sequences, fragments of polynucleotide or polypeptide sequences, variants of polynucleotide or polypeptide sequences, sense-oriented nucleotide sequences, antisense-oriented nucleotide sequences, the complement of a sense- or antisense-oriented nucleotide sequence, inverted regions of nucleotide sequences, hairpins of nucleotide sequences, double-stranded nucleotide sequences, single-stranded nucleotide sequences, combinations thereof, and the like. The term “polynucleotide sequence” includes sequences of RNA, DNA, chemically modified nucleic acids, nucleic acid analogs, combinations thereof, and the like.

Inhibitory sequences are designated by the name of the target gene product. Thus, a “ODC inhibitory sequence” refers to an inhibitory sequence that is capable of inhibiting the expression of an ODC gene involved in polyamine biosynthesis regulation in a plant, for example, at the level of transcription and/or translation, or which is capable of inhibiting the function of a gene product. When the phrase “capable of inhibiting” is used in the context of a polynucleotide inhibitory sequence, it is intended to mean that the inhibitory sequence itself exerts the inhibitory effect; or, where the inhibitory sequence encodes an inhibitory nucleotide molecule (for example, hairpin RNA, miRNA, or double-stranded RNA polynucleotides), or encodes an inhibitory polypeptide (e.g., a polypeptide that inhibits expression or function of the target gene product), following its transcription (for example, in the case of an inhibitory sequence encoding a hairpin RNA, miRNA, or double-stranded RNA polynucleotide) or its transcription and translation (in the case of an inhibitory sequence encoding an inhibitory polypeptide), the transcribed or translated product, respectively, exerts the inhibitory effect on the target gene product (e.g., inhibits expression or function of the target gene product).

A ODC inhibitory sequence disclosed can be a sequence triggering gene silencing via any silencing pathway or mechanism known in the art, including, but not limited to, sense suppression/cosuppression, antisense suppression, double-stranded RNA (dsRNA) interference, hairpin RNA interference and intron-containing hairpin RNA interference, amplicon-mediated interference, ribozymes, small interfering RNA, artificial or synthetic microRNA, and artificial trans-acting siRNA. An ODC inhibitory sequence may range from at least about 20 nucleotides, about 50 nucleotides, about 70 nucleotides, about 100 nucleotides, about 150 nucleotides, about 200 nucleotides, about 250 nucleotides, about 300 nucleotides, about 350 nucleotides, about 400 nucleotides, and up to the full-length polynucleotide encoding the proteins of the present disclosure, depending upon the desired outcome. In an aspect, a ODC inhibitory sequence can be a fragment of between about 50 and about 400 nucleotides, between about 70 and about 350 nucleotides, between about 90 and about 325 nucleotides, between about 90 and about 300 nucleotides, between about 90 and about 275 nucleotides, between about 100 and about 400 nucleotides, between about 100 and about 350 nucleotides, between about 100 and about 325 nucleotides, between about 100 and about 300 nucleotides, between about 125 and about 300 nucleotides, or between about 125 and about 275 nucleotides in length.

The use of the term “polynucleotide” is not intended to limit the present disclosure to polynucleotides comprising DNA. Those of ordinary skill in the art will recognize that polynucleotides can comprise ribonucleotides and combinations of ribonucleotides and deoxyribonucleotides. Such deoxyribonucleotides and ribonucleotides include both naturally occurring molecules and synthetic analogues. The polynucleotides of the present disclosure also encompass all forms of sequences including, but not limited to, single-stranded forms, double-stranded forms, hairpins, stem-and-loop structures, and the like.

In an aspect, the present disclosure provides recombinant DNA constructs comprising a promoter that is functional in a tobacco cell and operably linked to a polynucleotide that encodes an RNA molecule capable of binding to an RNA encoding a polypeptide having an amino acid sequence at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, at least 98%, at least 99% identical to an amino acid sequence selected from the group consisting of SEQ ID NOs: 23 to 28, and fragments thereof, and where the RNA molecule suppresses the expression of the polypeptide. In an aspect, the RNA molecule is selected from the group consisting of a microRNA, an siRNA, and a trans-acting siRNA. In another aspect, the recombinant DNA construct encodes a double stranded RNA. Also provided are transgenic tobacco plants or part thereof, cured tobacco material, or tobacco products comprising these recombinant DNA constructs. In an aspect, these transgenic plants, cured tobacco material, or tobacco products comprise a lower level of nicotine compared to a control tobacco plant without the recombinant DNA construct. Further provided are methods of reducing the nicotine level of a tobacco plant, the method comprising transforming a tobacco plant with any of these recombinant DNA constructs.

As used herein, “operably linked” refers to a functional linkage between two or more elements. For example, an operable linkage between a polynucleotide of interest and a regulatory sequence (e.g., a promoter) is a functional link that allows for expression of the polynucleotide of interest. Operably linked elements may be contiguous or non-contiguous.

As used herein and when used in reference to a sequence, “heterologous” refers to a sequence that originates from a foreign species, or, if from the same species, is substantially modified from its native form in composition and/or genomic location by deliberate human intervention. The term also is applicable to nucleic acid constructs, also referred to herein as “polynucleotide constructs” or “nucleotide constructs.” In this manner, a “heterologous” nucleic acid construct is intended to mean a construct that originates from a foreign species, or, if from the same species, is substantially modified from its native form in composition and/or genomic location by deliberate human intervention. Heterologous nucleic acid constructs include, but are not limited to, recombinant nucleotide constructs that have been introduced into a plant or plant part thereof, for example, via transformation methods or subsequent breeding of a transgenic plant with another plant of interest. In an aspect, a promoter used is heterologous to the sequence driven by the promoter. In another aspect, a promoter used is heterologous to tobacco. In a further aspect, a promoter used is native to tobacco.

In an aspect, a modified tobacco plant described is a cisgenic plant. As used herein, “cisgenesis” or “cisgenic” refers to genetic modification of a plant, plant cell, or plant genome in which all components (e.g., promoter, donor nucleic acid, selection gene) have only plant origins (i.e., no non-plant origin components are used). In an aspect, a modified plant, plant cell, or plant genome provided is cisgenic. Cisgenic plants, plant cells, and plant genomes provided can lead to ready-to-use tobacco lines. In another aspect, a modified tobacco plant provided comprises no non-tobacco genetic material or sequences.

As used herein, “gene expression” refers to the biosynthesis or production of a gene product, including the transcription and/or translation of the gene product.

In an aspect, recombinant DNA constructs or expression cassettes can also comprise a selectable marker gene for the selection of transgenic cells. Selectable marker genes include, but are not limited to, genes encoding antibiotic resistance, such as those encoding neomycin phosphotransferase II (NEO) and hygromycin phosphotransferase (HPT), as well as genes conferring resistance to herbicidal compounds, such as glufosinate ammonium, bromoxynil, imidazolinones, and 2,4-dichlorophenoxyacetate (2,4-D). Additional selectable markers include phenotypic markers such as β-galactosidase and fluorescent proteins such as green fluorescent protein (GFP).

In an aspect, recombinant DNA constructs or expression cassettes comprise a promoter selected from the group consisting of a constitutive promoter, an inducible promoter, and a tissue-preferred promoter (for example, a leaf-specific or root-specific promoter). Exemplary constitutive promoters include the core promoter of the Rsyn7 promoter and other constitutive promoters disclosed in U.S. Pat. No. 6,072,050; the core CaMV 35S promoter (Odell et al. (1985) Nature 313:810-812); ubiquitin (Christensen et al. (1989) Plant Mol. Biol. 12:619-632 and Christensen et al. (1992) Plant Mol. Biol. 18:675-689); pEMU (Last et al. (1991) Theor. Appl. Genet. 81:581-588); MAS (Velten et al. (1984) EMBO J 3:2723-2730); ALS promoter (U.S. Pat. No. 5,659,026), and the like. Exemplary chemical-inducible promoters include the tobacco PR-la promoter, which is activated by salicylic acid. Other chemical-inducible promoters of interest include steroid-responsive promoters (see, for example, the glucocorticoid-inducible promoter in Schena et al. (1991) Proc. Natl. Acad. Sci. USA 88:10421-10425 and McNellis et al. (1998) Plant J. 14(2):247-257) and tetracycline-inducible promoters (see, for example, Gatz et al. (1991) Mol. Gen. Genet. 227:229-237, and U.S. Pat. Nos. 5,814,618 and 5,789,156). Additional exemplary promoters that can be used are those responsible for heat-regulated gene expression, light-regulated gene expression (for example, the pea rbcS-3A; the maize rbcS promoter; the chlorophyll alb-binding protein gene found in pea; or the Arabssu promoter), hormone-regulated gene expression (for example, the abscisic acid (ABA) responsive sequences from the Em gene of wheat; the ABA-inducible HVA1 and HVA22, and rd29A promoters of barley and Arabidopsis; and wound-induced gene expression (for example, of wunl), organ specific gene expression (for example, of the tuber-specific storage protein gene; the 23-kDa zein gene from maize described by; or the French bean (β-phaseolin gene), or pathogen-inducible promoters (for example, the PR-1, prp-1, or (β-1,3 glucanase promoters, the fungal-inducible wirla promoter of wheat, and the nematode-inducible promoters, TobRB7-5A and Hmg-1, of tobacco arid parsley, respectively).

In an aspect, a tobacco plant provided further comprises increased or reduced expression of activity of genes involved in nicotine biosynthesis or transport. Genes involved in nicotine biosynthesis include, but are not limited to, arginine decarboxylase (ADC), methylputrescine oxidase (MPO), NADH dehydrogenase, ornithine decarboxylase (ODC), phosphoribosylanthranilate isomerase (PRAI), putrescine N-methyltransferase (PMT), quinolate phosphoribosyl transferase (QPT), and S-adenosyl-methionine synthetase (SAMS). Nicotine Synthase, which catalyzes the condensation step between a nicotinic acid derivative and methylpyrrolinium cation, has not been elucidated although two candidate genes (A622 and NBB1) have been proposed. See US 2007/0240728 A1 and US 2008/0120737A1. A622 encodes an isoflavone reductase-like protein. In addition, several transporters may be involved in the translocation of nicotine. A transporter gene, named MATE, has been cloned and characterized (Morita et al., PNAS 106:2447-52 (2009)).

In an aspect, a tobacco plant provided further comprises an increased or reduced level of mRNA, protein, or both of one or more genes encoding a product selected from the group consisting of PMT, MPO, QPT, ADC, ODC, PRAI, SAMS, BBL, MATE, A622, and NBB1, compared to a control tobacco plant. In another aspect, a tobacco plants provided further comprises a transgene directly suppressing the expression of one or more genes encoding a product selected from the group consisting of PMT, MPO, QPT, ADC, ODC, PRAI, SAMS, BBL, MATE, A622, and NBB1. In another aspect, a tobacco plant provided further comprises a transgene or mutation suppressing the expression or activity of one or more genes encoding a product selected from the group consisting of PMT, MPO, QPT, ADC, ODC, PRAI, SAMS, BBL, MATE, A622, and NBB1. In another aspect, a tobacco plant provided further comprises a transgene overexpressing one or more genes encoding a product selected from the group consisting of PMT, MPO, QPT, ADC, ODC, PRAI, SAMS, BBL, MATE, A622, and NBB1.

Also disclosed are the transformation of tobacco plants with recombinant constructs or expression cassettes described using any suitable transformation methods known in the art. Methods for introducing polynucleotide sequences into tobacco plants are known in the art and include, but are not limited to, stable transformation methods, transient transformation methods, and virus-mediated methods. “Stable transformation” refers to transformation where the nucleotide construct of interest introduced into a plant integrates into the genome of the plant and is capable of being inherited by the progeny thereof. “Transient transformation” is intended to mean that a sequence is introduced into the plant and is only temporally expressed or is only transiently present in the plant.

Suitable methods of introducing polynucleotides into plant cells of the present disclosure include microinjection (Crossway et al. (1986) Biotechniques 4:320-334), electroporation (Shillito et al. (1987) Meth. Enzymol. 153:313-336; Riggs et al. (1986) Proc. Natl. Acad. Sci. USA 83:5602-5606), Agrobacterium-mediated transformation (U.S. Pat. Nos. 5,104,310, 5,149,645, 5,177,010, 5,231,019, 5,463,174, 5,464,763, 5,469,976, 4,762,785, 5,004,863, 5,159,135, 5,563,055, and 5,981,840), direct gene transfer (Paszkowski et al. (1984) EMBO J. 3:2717-2722), and ballistic particle acceleration (see, for example, U.S. Pat. Nos. 4,945,050, 5,141,131, 5,886,244, 5,879,918, and 5,932,782; Tomes et al. (1995) in Plant Cell, Tissue, and Organ Culture Fundamental Methods, ed. Gamborg and Phillips (Springer-Verlag, Berlin); McCabe et al. (1988) Biotechnology 6:923-926). Also see Weissinger et al. (1988) Ann. Rev. Genet. 22:421-477; Christou et al. (1988) Plant Physiol. 87:671-674 (soybean); McCabe et al. (1988) Bio/Technology 6:923-926 (soybean); Finer and McMullen (1991) In Vitro Cell Dev. Biol. 27P: 175-182 (soybean); Singh et al. (1998) Theor. Appl. Genet. 96:319-324 (soybean); De Wet et al. (1985) in The Experimental Manipulation of Ovule Tissues, ed. Chapman et al. (Longman, N.Y.), pp. 197-209 (pollen); Kaeppler et al. (1990) Plant Cell Reports 9:415-418 and Kaeppler et al. (1992) Theor. Appl. Genet. 84:560-566 (whisker-mediated transformation); D'Halluin et al. (1992) Plant Cell 4:1495-1505 (electroporation).

In another aspect, recombinant constructs or expression cassettes may be introduced into plants by contacting plants with a virus or viral nucleic acids. Generally, such methods involve incorporating an expression cassette of the present disclosure within a viral DNA or RNA molecule. It is recognized that promoters for use in expression cassettes also encompass promoters utilized for transcription by viral RNA polymerases. Methods for introducing polynucleotides into plants and expressing a protein encoded therein, involving viral DNA or RNA molecules, are known in the art. See, for example, U.S. Pat. Nos. 5,889,191, 5,889,190, 5,866,785, 5,589,367, 5,316,931, and Porta et al. (1996) Molecular Biotechnology 5:209-221.

Any plant tissue that can be subsequently propagated using clonal methods, whether by organogenesis or embryogenesis, may be transformed with a recombinant construct or an expression cassette. By “organogenesis” in intended the process by which shoots and roots are developed sequentially from meristematic centers. By “embryogenesis” is intended the process by which shoots and roots develop together in a concerted fashion (not sequentially), whether from somatic cells or gametes. Exemplary tissues that are suitable for various transformation protocols described include, but are not limited to, callus tissue, existing meristematic tissue (e.g., apical meristems, axillary buds, and root meristems) and induced meristem tissue (e.g., cotyledon meristem and hypocotyl meristem), hypocotyls, cotyledons, leaf disks, pollen, embryos, and the like.

In an aspect, a tobacco plant provided is from a tobacco type selected from the group consisting of flue-cured tobacco, air-cured tobacco, dark air-cured tobacco, dark fire-cured tobacco, Galpao tobacco, and Oriental tobacco. In another aspect, a tobacco plant provided is from a tobacco type selected from the group consisting of Burley tobacco, Maryland tobacco, and dark tobacco.

In an aspect, a tobacco plant provided is in a flue-cured tobacco background or exhibits one or more flue-cured tobacco characteristic described here. Flue-cured tobaccos (also called Virginia or bright tobaccos) amount to approximately 40% of world tobacco production. Flue-cured tobaccos are often also referred to as “bright tobacco” because of the golden-yellow to deep-orange color it reaches during curing. Flue-cured tobaccos have a light, bright aroma and taste. Flue-cured tobaccos are generally high in sugar and low in oils. Major flue-cured tobacco growing countries are Argentina, Brazil, China, India, Tanzania and the U.S. In an aspect, a low-alkaloid or low-nicotine tobacco plant or seed provided is in a flue-cured tobacco background selected from the group consisting of CC 13, CC 27, CC 33, CC 37, CC 65, CC 67, CC 700, GF 318, GL 338, GL 368, GL 939, K 346, K 399, K326, NC 102, NC 196, NC 291, NC 297, NC 299, NC 471, NC 55, NC 606, NC 71, NC 72, NC 92, PVH 1118, PVH 1452, PVH 2110, SPEIGHT 168, SPEIGHT 220, SPEIGHT 225, SPEIGHT 227, SPEIGHT 236, and any variety essentially derived from any one of the foregoing varieties. In another aspect, a low-alkaloid or low-nicotine tobacco plant or seed provided is in a flue-cured tobacco background selected from the group consisting of Coker 48, Coker 176, Coker 371-Gold, Coker 319, Coker 347, GL 939, K 149, K326, K 340, K 346, K 358, K 394, K 399, K 730, NC 27NF, NC 37NF, NC 55, NC 60, NC 71, NC 72, NC 82, NC 95, NC 297, NC 606, NC 729, NC 2326, McNair 373, McNair 944, Ox 207, Ox 414 NF, Reams 126, Reams 713, Reams 744, RG 8, RG 11, RG 13, RG 17, RG 22, RG 81, RG H4, RG H51, Speight H-20, Speight G-28, Speight G-58, Speight G-70, Speight G-108, Speight G-111, Speight G-117, Speight 168, Speight 179, Speight NF-3, Va 116, Va 182, and any variety essentially derived from any one of the foregoing varieties. See WO 2004/041006 A1. In a further aspect, low-alkaloid or low-nicotine tobacco plants, seeds, hybrids, varieties, or lines are in any flue cured background selected from the group consisting of K326, K346, and NC196.

In an aspect, a tobacco plant provided is in an air-cured tobacco background or exhibits one or more air-cured tobacco characteristic described here. Air-cured tobaccos include Burley, Md., and dark tobaccos. The common factor is that curing is primarily without artificial sources of heat and humidity. Burley tobaccos are light to dark brown in color, high in oil, and low in sugar. Burley tobaccos are air-cured in barns. Major Burley growing countries are Argentina, Brazil, Italy, Malawi, and the U.S. Maryland tobaccos are extremely fluffy, have good burning properties, low nicotine and a neutral aroma. Major Maryland growing countries include the U.S. and Italy. In an aspect, a low-alkaloid or low-nicotine tobacco plant or seed provided is in a Burley tobacco background selected from the group consisting of Clay 402, Clay 403, Clay 502, Ky 14, Ky 907, Ky 910, Ky 8959, NC 2, NC 3, NC 4, NC 5, NC 2000, TN 86, TN 90, TN 97, R 610, R 630, R 711, R 712, NCBH 129, Bu 21×Ky 10, HBO4P, Ky 14×L 8, Kt 200, Newton 98, Pedigo 561, Pf561 and Va 509. In a further aspect, low-alkaloid or low-nicotine tobacco plants, seeds, hybrids, varieties, or lines are in any Burley background selected from the group consisting of TN 90, KT 209, KT 206, KT212, and HB 4488. In another aspect, a low-alkaloid or low-nicotine tobacco plant or seed provided is in a Maryland tobacco background selected from the group consisting of Md 10, Md 40, Md 201, Md 609, Md 872 and Md 341.

In an aspect, a tobacco plant provided is in a dark air-cured tobacco background or exhibits one or more dark air-cured tobacco characteristic described here. Dark air-cured tobaccos are distinguished from other types primarily by its curing process which gives dark air-cured tobacco its medium- to dark-brown color and distinct aroma. Dark air-cured tobaccos are mainly used in the production of chewing tobacco and snuff. In an aspect, a low-alkaloid or low-nicotine tobacco plant or seed provided is in a dark air-cured tobacco background selected from the group consisting of Sumatra, Jatim, Dominican Cubano, Besuki, One sucker, Green River, Va. sun-cured, and Paraguan Passado.

In an aspect, a tobacco plant provided is in a dark fire-cured tobacco background or exhibits one or more dark fire-cured tobacco characteristic described here. Dark fire-cured tobaccos are generally cured with low-burning wood fires on the floors of closed curing barns. Their leaves have low sugar content but high nicotine content. Dark fire-cured tobaccos are used for making pipe blends, cigarettes, chewing tobacco, snuff and strong-tasting cigars. Major growing regions for dark fire-cured tobaccos are Tennessee, Kentucky, and Virginia, USA. In an aspect, a low-alkaloid or low-nicotine tobacco plant or seed provided is in a dark fire-cured tobacco background selected from the group consisting of Narrow Leaf Madole, Improved Madole, Tom Rosson Madole, Newton's VH Madole, Little Crittenden, Green Wood, Little Wood, Small Stalk Black Mammoth, DT 508, DT 518, DT 592, KY 171, DF 911, DF 485, TN D94, TN D950, VA 309, and VA 359.

In an aspect, a tobacco plant provided is in an Oriental tobacco background or exhibits one or more Oriental tobacco characteristic described here. Oriental tobaccos are also referred to as Greek, aroma and Turkish tobaccos due to the fact that they are typically grown in eastern Mediterranean regions such as Turkey, Greece, Bulgaria, Macedonia, Syria, Lebanon, Italy, and Romania. The small plant and leaf size, characteristic of today's Oriental varieties, as well as its unique aroma properties are a result of the plant's adaptation to the poor soil and stressful climatic conditions in which it develop over many past centuries. In an aspect, a low-alkaloid or low-nicotine tobacco plant or seed provided is in an Oriental tobacco background selected from the group consisting of Izmir, Katerini, Samsun, Basma and Krumovgrad, Trabzon, Thesalian, Tasova, Sinop, Izmit, Hendek, Edirne, Semdinli, Adiyanman, Yayladag, Iskenderun, Duzce, Macedonian, Mavra, Prilep, Bafra, Bursa, Bucak, Bitlis, Balikesir, and any variety essentially derived from any one of the foregoing varieties.

In an aspect, low-alkaloid or low-nicotine tobacco plants, seeds, hybrids, varieties, or lines are essentially derived from or in the genetic background of BU 64, CC 101, CC 200, CC 27, CC 301, CC 400, CC 500, CC 600, CC 700, CC 800, CC 900, Coker 176, Coker 319, Coker 371 Gold, Coker 48, CU 263, DF911, Galpao tobacco, GL 26H, GL 350, GL 600, GL 737, GL 939, GL 973, HB 04P, K 149, K 326, K 346, K 358, K394, K 399, K 730, KDH 959, KT 200, KT204LC, KY 10, KY 14, KY 160, KY 17, KY 171, KY 907, KY907LC, KTY14×L8 LC, Little Crittenden, McNair 373, McNair 944, msKY 14×L8, Narrow Leaf Madole, NC 100, NC 102, NC 2000, NC 291, NC 297, NC 299, NC 3, NC 4, NC 5, NC 6, NC7, NC 606, NC 71, NC 72, NC 810, NC BH 129, NC 2002, Neal Smith Madole, OXFORD 207, ‘Perique’ tobacco, PVH03, PVH09, PVH19, PVH50, PVH51, R 610, R 630, R 7-11, R 7-12, RG 17, RG 81, RG H51, RGH 4, RGH 51, RS 1410, Speight 168, Speight 172, Speight 179, Speight 210, Speight 220, Speight 225, Speight 227, Speight 234, Speight G-28, Speight G-70, Speight H-6, Speight H20, Speight NF3, TI 1406, TI 1269, TN 86, TN86LC, TN 90, TN 97, TN97LC, TN D94, TN D950, TR (Tom Rosson) Madole, VA 309, or VA359, Maryland 609, HB3307PLC, HB4488PLC, KT206LC, KT209LC, KT210LC, KT212LC, R610LC, PVH2310, NC196, KTD14LC, KTD6LC, KTD8LC, PD7302LC, PD7305LC, PD7309LC, PD7318LC, PD7319LC, PD7312LC, ShireyLC, or any commercial tobacco variety according to standard tobacco breeding techniques known in the art.

All foregoing mentioned specific varieties of dark air-cured, Burley, Md., dark fire-cured, or Oriental type are listed only for exemplary purposes. Any additional dark air-cured, Burley, Md., dark fire-cured, Oriental varieties are also contemplated in the present application.

Also provided are populations of tobacco plants described. In an aspect, a population of tobacco plants has a planting density of between about 5,000 and about 8000, between about 5,000 and about 7,600, between about 5,000 and about 7,200, between about 5,000 and about 6,800, between about 5,000 and about 6,400, between about 5,000 and about 6,000, between about 5,000 and about 5,600, between about 5,000 and about 5,200, between about 5,200 and about 8,000, between about 5,600 and about 8,000, between about 6,000 and about 8,000, between about 6,400 and about 8,000, between about 6,800 and about 8,000, between about 7,200 and about 8,000, or between about 7,600 and about 8,000 plants per acre. In another aspect, a population of tobacco plants is in a soil type with low to medium fertility.

Also provided are containers of seeds from tobacco plants described. A container of tobacco seeds of the present disclosure may contain any number, weight, or volume of seeds. For example, a container can contain at least, or greater than, about 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 1500, 2000, 2500, 3000, 3500, 4000 or more seeds. Alternatively, the container can contain at least, or greater than, about 1 ounce, 5 ounces, 10 ounces, 1 pound, 2 pounds, 3 pounds, 4 pounds, 5 pounds or more seeds. Containers of tobacco seeds may be any container available in the art. By way of non-limiting example, a container may be a box, a bag, a packet, a pouch, a tape roll, a tube, or a bottle.

Also provided is cured tobacco material made from a low-alkaloid or low-nicotine tobacco plant described. Further provided is cured tobacco material made from a tobacco plant described with higher levels of total alkaloid or nicotine.

“Curing” is the aging process that reduces moisture and brings about the destruction of chlorophyll giving tobacco leaves a golden color and by which starch is converted to sugar. Cured tobacco therefore has a higher reducing sugar content and a lower starch content compared to harvested green leaf. In an aspect, green leaf tobacco provided can be cured using conventional means, e.g., flue-cured, barn-cured, fire-cured, air-cured or sun-cured. See, for example, Tso (1999, Chapter 1 in Tobacco, Production, Chemistry and Technology, Davis & Nielsen, eds., Blackwell Publishing, Oxford) for a description of different types of curing methods. Cured tobacco is usually aged in a wooden drum (e.g., a hogshead) or cardboard cartons in compressed conditions for several years (e.g., two to five years), at a moisture content ranging from 10% to about 25%. See, U.S. Pat. Nos. 4,516,590 and 5,372,149. Cured and aged tobacco then can be further processed. Further processing includes conditioning the tobacco under vacuum with or without the introduction of steam at various temperatures, pasteurization, and fermentation. Fermentation typically is characterized by high initial moisture content, heat generation, and a 10 to 20% loss of dry weight. See, e.g., U.S. Pat. Nos. 4,528,993, 4,660,577, 4,848,373, 5,372,149; U.S. Publication No. 2005/0178398; and Tso (1999, Chapter 1 in Tobacco, Production, Chemistry and Technology, Davis & Nielsen, eds., Blackwell Publishing, Oxford). Cure, aged, and fermented tobacco can be further processed (e.g., cut, shredded, expanded, or blended). See, for example, U.S. Pat. Nos. 4,528,993; 4,660,577; and 4,987,907. In an aspect, the cured tobacco material of the present disclosure is sun-cured. In another aspect, the cured tobacco material of the present disclosure is flue-cured, air-cured, or fire-cured.

Tobacco material obtained from the tobacco lines, varieties or hybrids of the present disclosure can be used to make tobacco products. As used herein, “tobacco product” is defined as any product made or derived from tobacco that is intended for human use or consumption.

Tobacco products provided include, without limitation, cigarette products (e.g., cigarettes and bidi cigarettes), cigar products (e.g., cigar wrapping tobacco and cigarillos), pipe tobacco products, products derived from tobacco, tobacco-derived nicotine products, smokeless tobacco products (e.g., moist snuff, dry snuff, and chewing tobacco), films, chewables, tabs, shaped parts, gels, consumable units, insoluble matrices, hollow shapes, reconstituted tobacco, expanded tobacco, and the like. See, e.g., U.S. Patent Publication No. US 2006/0191548.

As used herein, “cigarette” refers a tobacco product having a “rod” and “filler”. The cigarette “rod” includes the cigarette paper, filter, plug wrap (used to contain filtration materials), tipping paper that holds the cigarette paper (including the filler) to the filter, and all glues that hold these components together. The “filler” includes (1) all tobaccos, including but not limited to reconstituted and expanded tobacco, (2) non-tobacco substitutes (including but not limited to herbs, non-tobacco plant materials and other spices that may accompany tobaccos rolled within the cigarette paper), (3) casings, (4) flavorings, and (5) all other additives (that are mixed into tobaccos and substitutes and rolled into the cigarette).

As used herein, “reconstituted tobacco” refers to a part of tobacco filler made from tobacco dust and other tobacco scrap material, processed into sheet form and cut into strips to resemble tobacco. In addition to the cost savings, reconstituted tobacco is very important for its contribution to cigarette taste from processing flavor development using reactions between ammonia and sugars.

As used herein, “expanded tobacco” refers to a part of tobacco filler which is processed through expansion of suitable gases so that the tobacco is “puffed” resulting in reduced density and greater filling capacity. It reduces the weight of tobacco used in cigarettes.

Tobacco products derived from plants of the present disclosure also include cigarettes and other smoking articles, particularly those smoking articles including filter elements, where the rod of smokable material includes cured tobacco within a tobacco blend. In an aspect, a tobacco product of the present disclosure is selected from the group consisting of a cigarillo, a non-ventilated recess filter cigarette, a vented recess filter cigarette, a cigar, snuff, pipe tobacco, cigar tobacco, cigarette tobacco, chewing tobacco, leaf tobacco, hookah tobacco, shredded tobacco, and cut tobacco. In another aspect, a tobacco product of the present disclosure is a smokeless tobacco product. Smokeless tobacco products are not combusted and include, but not limited to, chewing tobacco, moist smokeless tobacco, snus, and dry snuff. Chewing tobacco is coarsely divided tobacco leaf that is typically packaged in a large pouch-like package and used in a plug or twist. Moist smokeless tobacco is a moist, more finely divided tobacco that is provided in loose form or in pouch form and is typically packaged in round cans and used as a pinch or in a pouch placed between an adult tobacco consumer's cheek and gum. Snus is a heat treated smokeless tobacco. Dry snuff is finely ground tobacco that is placed in the mouth or used nasally. In a further aspect, a tobacco product of the present disclosure is selected from the group consisting of loose leaf chewing tobacco, plug chewing tobacco, moist snuff, and nasal snuff. In yet another aspect, a tobacco product of the present disclosure is selected from the group consisting of an electronically heated cigarette, an e-cigarette, an electronic vaporing device.

In an aspect, a tobacco product of the present disclosure can be a blended tobacco product. In one aspect, a blended tobacco product comprises cured tobacco materials. In an aspect, a cured tobacco material constitutes about at least 10%, at least 15%, at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, or at least 95% of cured tobacco in a tobacco blend by weight. In one aspect, a cured tobacco material constitutes about at least 10%, at least 15%, at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, or at least 95% of cured tobacco in a tobacco blend by volume.

In an aspect, a tobacco product of the present disclosure can be a low nicotine tobacco product. In a further aspect, a tobacco product of the present disclosure may comprise nornicotine at a level of less than about 3 mg/g. For example, the nornicotine content in such a product can be about 3.0 mg/g, 2.5 mg/g, 2.0 mg/g, 1.5 mg/g, 1.0 mg/g, 750 pg/g, 500 pg/g, 250 pg/g, 100 pg/g, 75 pg/g, 50 pg/g, 25 pg/g, 10 pg/g, 7.0 pg/g, 5.0 pg/g, 4.0 pg/g, 2.0 pg/g, 1.0 pg/g, 0.5 pg/g, 0.4 pg/g, 0.2 pg/g, 0.1 pg/g, 0.05 pg/g, 0.01 pg/g, or undetectable.

In an aspect, cured tobacco material or tobacco products provided comprise an average nicotine or total alkaloid level selected from the group consisting of about 0.01%, 0.02%, 0.05%, 0.75%, 0.1%, 0.15%, 0.2%, 0.3%, 0.35%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3%, 3.1%, 3.2%, 3.3%, 3.4%, 3.5%, 3.6%, 3.7%, 3.8%, 3.9%, 4%, 5%, 6%, 7%, 8%, and 9% on a dry weight basis. In another aspect, cured tobacco material or tobacco products provided comprise an average nicotine or total alkaloid level selected from the group consisting of about between 0.01% and 0.02%, between 0.02% and 0.05%, between 0.05% and 0.75%, between 0.75% and 0.1%, between 0.1% and 0.15%, between 0.15% and 0.2%, between 0.2% and 0.3%, between 0.3% and 0.35%, between 0.35% and 0.4%, between 0.4% and 0.5%, between 0.5% and 0.6%, between 0.6% and 0.7%, between 0.7% and 0.8%, between 0.8% and 0.9%, between 0.9% and 1%, between 1% and 1.1%, between 1.1% and 1.2%, between 1.2% and 1.3%, between 1.3% and 1.4%, between 1.4% and 1.5%, between 1.5% and 1.6%, between 1.6% and 1.7%, between 1.7% and 1.8%, between 1.8% and 1.9%, between 1.9% and 2%, between 2% and 2.1%, between 2.1% and 2.2%, between 2.2% and 2.3%, between 2.3% and 2.4%, between 2.4% and 2.5%, between 2.5% and 2.6%, between 2.6% and 2.7%, between 2.7% and 2.8%, between 2.8% and 2.9%, between 2.9% and 3%, between 3% and 3.1%, between 3.1% and 3.2%, between 3.2% and 3.3%, between 3.3% and 3.4%, between 3.4% and 3.5%, and between 3.5% and 3.6% on a dry weight basis. In a further aspect, cured tobacco material or tobacco products provided comprise an average nicotine or total alkaloid level selected from the group consisting of about between 0.01% and 0.1%, between 0.02% and 0.2%, between 0.03% and 0.3%, between 0.04% and 0.4%, between 0.05% and 0.5%, between 0.75% and 1%, between 0.1% and 1.5%, between 0.15% and 2%, between 0.2% and 3%, and between 0.3% and 3.5% on a dry weight basis.

The present disclosure also provides methods for breeding tobacco lines, cultivars, or varieties comprising a desirable level of total alkaloid or nicotine, e.g., low nicotine or nicotine free. Breeding can be carried out via any known procedures. DNA fingerprinting, SNP mapping, haplotype mapping or similar technologies may be used in a marker-assisted selection (MAS) breeding program to transfer or breed a desirable trait or allele into a tobacco plant. For example, a breeder can create segregating populations in a F₂ or backcross generation using F1 hybrid plants or further crossing the F1 hybrid plants with other donor plants with an agronomically desirable genotype. Plants in the F₂ or backcross generations can be screened for a desired agronomic trait or a desirable chemical profile using one of the techniques known in the art or listed herein. Depending on the expected inheritance pattern or the MAS technology used, self-pollination of selected plants before each cycle of backcrossing to aid identification of the desired individual plants can be performed. Backcrossing or other breeding procedure can be repeated until the desired phenotype of the recurrent parent is recovered. A recurrent parent in the present disclosure can be a flue-cured variety, a Burley variety, a dark air-cured variety, a dark fire-cured variety, or an Oriental variety. Other breeding techniques can be found, for example, in Wernsman, E. A., and Rufty, R. C. 1987. Chapter Seventeen. Tobacco. Pages 669-698 In: Cultivar Development. Crop Species. W. H. Fehr (ed.), MacMillan Publishing Go., Inc., New York, N.Y., incorporated herein by reference in their entirety.

Results of a plant breeding program using the tobacco plants described includes useful lines, cultivars, varieties, progeny, inbreds, and hybrids of the present disclosure. As used herein, the term “variety” refers to a population of plants that share constant characteristics which separate them from other plants of the same species. A variety is often, although not always, sold commercially. While possessing one or more distinctive traits, a variety is further characterized by a very small overall variation between individuals within that variety. A “pure line” variety may be created by several generations of self-pollination and selection, or vegetative propagation from a single parent using tissue or cell culture techniques. A variety can be essentially derived from another line or variety. As defined by the International Convention for the Protection of New Varieties of Plants (Dec. 2, 1961, as revised at Geneva on Nov. 10, 1972; on Oct. 23, 1978; and on Mar. 19, 1991), a variety is “essentially derived” from an initial variety if: a) it is predominantly derived from the initial variety, or from a variety that is predominantly derived from the initial variety, while retaining the expression of the essential characteristics that result from the genotype or combination of genotypes of the initial variety; b) it is clearly distinguishable from the initial variety; and c) except for the differences which result from the act of derivation, it conforms to the initial variety in the expression of the essential characteristics that result from the genotype or combination of genotypes of the initial variety. Essentially derived varieties can be obtained, for example, by the selection of a natural or induced mutant, a somaclonal variant, a variant individual from plants of the initial variety, backcrossing, or transformation. A first tobacco variety and a second tobacco variety from which the first variety is essentially derived, are considered as having essentially identical genetic background. A “line” as distinguished from a variety most often denotes a group of plants used non-commercially, for example in plant research. A line typically displays little overall variation between individuals for one or more traits of interest, although there may be some variation between individuals for other traits.

As used herein, “locus” is a chromosome region where a polymorphic nucleic acid, trait determinant, gene, or marker is located. The loci of this disclosure comprise one or more polymorphisms in a population; e.g., alternative alleles are present in some individuals. As used herein, “allele” refers to an alternative nucleic acid sequence at a particular locus. The length of an allele can be as small as 1 nucleotide base, but is typically larger. For example, a first allele can occur on one chromosome, while a second allele occurs on a second homologous chromosome, e.g., as occurs for different chromosomes of a heterozygous individual, or between different homozygous or heterozygous individuals in a population. As used herein, the term “chromosome interval” designates a contiguous linear span of genomic DNA that resides on a single chromosome.

As used herein, “introgression” or “introgress” refers to the transmission of a desired allele of a genetic locus from one genetic background to another.

As used herein, “crossed” or “cross” means to produce progeny via fertilization (e.g. cells, seeds or plants) and includes crosses between plants (sexual) and self fertilization (selfing).

As used herein, “backcross” and “backcrossing” refer to the process whereby a progeny plant is repeatedly crossed back to one of its parents. In a backcrossing scheme, the “donor” parent refers to the parental plant with the desired gene or locus to be introgressed. The “recipient” parent (used one or more times) or “recurrent” parent (used two or more times) refers to the parental plant into which the gene or locus is being introgressed. The initial cross gives rise to the F1 generation. The term “BC1” refers to the second use of the recurrent parent, “BC2” refers to the third use of the recurrent parent, and so on. In an aspect, a backcross is performed repeatedly, with a progeny individual of each successive backcross generation being itself backcrossed to the same parental genotype.

As used herein, “single gene converted” or “single gene conversion” refers to plants that are developed using a plant breeding technique known as backcrossing, or via genetic engineering, where essentially all of the desired morphological and physiological characteristics of a variety are recovered in addition to the single gene transferred into the variety via the backcrossing technique or via genetic engineering.

As used herein, “elite variety” means any variety that has resulted from breeding and selection for superior agronomic performance.

As used herein, “selecting” or “selection” in the context of marker-assisted selection or breeding refer to the act of picking or choosing desired individuals, normally from a population, based on certain pre-determined criteria.

As used herein, the term “trait” refers to one or more detectable characteristics of a cell or organism which can be influenced by genotype. The phenotype can be observable to the naked eye, or by any other means of evaluation known in the art, e.g., microscopy, biochemical analysis, genomic analysis, an assay for a particular disease tolerance, etc. In some cases, a phenotype is directly controlled by a single gene or genetic locus, e.g., a “single gene trait.” In other cases, a phenotype is the result of several genes.

It is understood that any tobacco plant of the present disclosure can further comprise additional agronomically desirable traits, for example, by transformation with a genetic construct or transgene using a technique known in the art. Without limitation, an example of a desired trait is herbicide resistance, pest resistance, disease resistance; high yield; high grade index value; curability; curing quality; mechanical harvestability; holding ability; leaf quality; height, plant maturation (e.g., early maturing, early to medium maturing, medium maturing, medium to late maturing, or late maturing); stalk size (e.g., a small, medium, or a large stalk); or leaf number per plant (e.g., a small (e.g., 5-10 leaves), medium (e.g., 11-15 leaves), or large (e.g., 16-21) number of leaves), or any combination. In an aspect, low-nicotine or nicotine-free tobacco plants or seeds disclosed comprise one or more transgenes expressing one or more insecticidal proteins, such as, for example, a crystal protein of Bacillus thuringiensis or a vegetative insecticidal protein from Bacillus cereus, such as VIP3 (see, for example, Estruch et al. (1997) Nat. Biotechnol.15:137). In another aspect, tobacco plants further comprise an introgressed trait conferring resistance to brown stem rot (U.S. Pat. No. 5,689,035) or resistance to cyst nematodes (U.S. Pat. No. 5,491,081).

The present disclosure also provides tobacco plants comprising an altered nicotine or total alkaloid level but having a yield comparable to the yield of corresponding initial tobacco plants without such a nicotine level alternation. In an aspect, a low-nicotine or nicotine-free tobacco variety provides a yield selected from the group consisting of about between 1200 and 3500, between 1300 and 3400, between 1400 and 3300, between 1500 and 3200, between 1600 and 3100, between 1700 and 3000, between 1800 and 2900, between 1900 and 2800, between 2000 and 2700, between 2100 and 2600, between 2200 and 2500, and between 2300 and 2400 lbs/acre. In another aspect, a low-nicotine or nicotine-free tobacco variety provides a yield selected from the group consisting of about between 1200 and 3500, between 1300 and 3500, between 1400 and 3500, between 1500 and 3500, between 1600 and 3500, between 1700 and 3500, between 1800 and 3500, between 1900 and 3500, between 2000 and 3500, between 2100 and 3500, between 2200 and 3500, between 2300 and 3500, between 2400 and 3500, between 2500 and 3500, between 2600 and 3500, between 2700 and 3500, between 2800 and 3500, between 2900 and 3500, between 3000 and 3500, and between 3100 and 3500 lbs/acre. In a further aspect, low-nicotine or nicotine-free tobacco plants provide a yield between 65% and 130%, between 70% and 130%, between 75% and 130%, between 80% and 130%, between 85% and 130%, between 90% and 130%, between 95% and 130%, between 100% and 130%, between 105% and 130%, between 110% and 130%, between 115% and 130%, or between 120% and 130% of the yield of a control plant having essentially identical genetic background except a nic1b mutation, a nic2 mutation, a Nic1b transgene, a Nic2 transgene, or combinations thereof. In a further aspect, low-nicotine or nicotine-free tobacco plants provide a yield between 70% and 125%, between 75% and 120%, between 80% and 115%, between 85% and 110%, or between 90% and 100% of the yield of a control plant having essentially identical genetic background except a nic1 mutation, a nic2 mutation, a Nic1 transgene, a Nic2 transgene, or combinations thereof.

In an aspect, a tobacco plant (e.g., a low-nicotine, nicotine-free, or low-alkaloid tobacco variety) does not exhibit one or more, two or more, three or more, or all of the LA BU21 traits selected from the group consisting of lower yield, delayed ripening and senescence, higher susceptibility to insect herbivory, increased polyamine content after topping, higher chlorophyll, more mesophyll cells per unit leaf area, and poor end-product quality after curing. In an aspect, a tobacco plant disclosed (e.g., a low-nicotine, nicotine-free, or low-alkaloid tobacco variety) does not exhibit two or more of the LA BU21 traits selected from the group consisting of lower yield, delayed ripening and senescence, higher susceptibility to insect herbivory, increased polyamine content after topping, higher chlorophyll, more mesophyll cells per unit leaf area, and poor end-product quality after curing. In an aspect, a tobacco plant disclosed (e.g., a low-nicotine, nicotine-free, or low-alkaloid tobacco variety) does not exhibit three or more of the LA BU21 traits selected from the group consisting of lower yield, delayed ripening and senescence, higher susceptibility to insect herbivory, increased polyamine content after topping, higher chlorophyll, more mesophyll cells per unit leaf area, and poor end-product quality after curing. In an aspect, a tobacco plant disclosed (e.g., a low-nicotine, nicotine-free, or low-alkaloid tobacco variety) exhibits at a lower level compared to LA BU21, LAFC53, or LN KY171, one or more, two or more, three or more, or all of the LA BU21 traits selected from the group consisting of lower yield, delayed ripening and senescence, higher susceptibility to insect herbivory, increased polyamine content after topping, higher chlorophyll, more mesophyll cells per unit leaf area, and poor end-product quality after curing. In an aspect, a tobacco plant disclosed (e.g., a low-nicotine, nicotine-free, or low-alkaloid tobacco variety) exhibits at a lower level compared to LA BU21, LAFC53, or LN KY171, two or more of the LA BU21 traits selected from the group consisting of lower yield, delayed ripening and senescence, higher susceptibility to insect herbivory, increased polyamine content after topping, higher chlorophyll, more mesophyll cells per unit leaf area, and poor end-product quality after curing. In an aspect, a tobacco plant disclosed (e.g., a low-nicotine, nicotine-free, or low-alkaloid tobacco variety) exhibits at a lower level compared to LA BU21, LAFC53, or LN KY171, three or more, or all of the LA BU21 traits selected from the group consisting of lower yield, delayed ripening and senescence, higher susceptibility to insect herbivory, increased polyamine content after topping, higher chlorophyll, more mesophyll cells per unit leaf area, and poor end-product quality after curing.

In an aspect, a modified tobacco plant (e.g., a low-nicotine, nicotine-free, or low-alkaloid tobacco variety) comprises a modification conferring a desired trait (e.g., low-nicotine, nicotine-free, or low-alkaloid) without substantially impacting a trait selected from the group consisting of yield, ripening and senescence, susceptibility to insect herbivory, polyamine content after topping, chlorophyll level, mesophyll cell number per unit leaf area, and end-product quality after curing.

In an aspect, a modified tobacco plant comprises a modification conferring a desired trait (e.g., low-nicotine, nicotine-free, or low-alkaloid) and further comprises a trait substantially comparable to an unmodified control plant, where the trait is selected from the group consisting of yield, ripening and senescence, susceptibility to insect herbivory, polyamine content after topping, chlorophyll level, mesophyll cell number per unit leaf area, and end-product quality after curing.

In an aspect, a modified tobacco plant comprises a modification conferring a desired trait (e.g., low-nicotine, nicotine-free, or low-alkaloid) and further comprises a yield which is more than 80%, more than 85%, more than 90%, more than 95%, more than 100%, more than 105%, more than 110%, more than 115%, more than 120%, more than 125%, more than 130%, more than 135%, or more than 140% relative to the yield of an unmodified control plant. In an aspect, a modified tobacco plant disclosed comprises a modification conferring a desired trait (e.g., low-nicotine, nicotine-free, or low-alkaloid) and further comprises a yield which is between 70% and 140%, between 75% and 135%, between 80% and 130%, between 85% and 125%, between 90% and 120%, between 95% and 115%, or between 100% and 110% relative to the yield of an unmodified control plant. In an aspect, a modified tobacco plant disclosed comprises a modification conferring a desired trait (e.g., low-nicotine, nicotine-free, or low-alkaloid) and further comprises a yield which is between 70% and 80%, between 75% and 85%, between 80% and 90%, between 85% and 95%, between 90% and 100%, between 95% and 105%, between 105% and 115%, between 110% and 120%, between 115% to 125%, between 120% and 130%, between 125 and 135%, or between 130% and 140% relative to the yield of an unmodified control plant.

In an aspect, a modified tobacco plant comprises a modification conferring a desired trait (e.g., low-nicotine, nicotine-free, or low-alkaloid) and further comprises a polyamine content after topping which is more than 80%, more than 85%, more than 90%, more than 95%, more than 100%, more than 105%, more than 110%, more than 115%, more than 120%, more than 125%, more than 130%, more than 135%, or more than 140% relative to the polyamine content after topping of an unmodified control plant. In an aspect, a modified tobacco plant disclosed comprises a modification conferring a desired trait (e.g., low-nicotine, nicotine-free, or low-alkaloid) and further comprises a polyamine content after topping which is between 70% and 140%, between 75% and 135%, between 80% and 130%, between 85% and 125%, between 90% and 120%, between 95% and 115%, or between 100% and 110% relative to the polyamine content after topping of an unmodified control plant. In an aspect, a modified tobacco plant disclosed comprises a modification conferring a desired trait (e.g., low-nicotine, nicotine-free, or low-alkaloid) and further comprises a polyamine content after topping which is between 70% and 80%, between 75% and 85%, between 80% and 90%, between 85% and 95%, between 90% and 100%, between 95% and 105%, between 105% and 115%, between 110% and 120%, between 115% to 125%, between 120% and 130%, between 125 and 135%, or between 130% and 140% relative to the polyamine content after topping of an unmodified control plant.

In an aspect, a modified tobacco plant comprises a modification conferring a desired trait (e.g., low-nicotine, nicotine-free, or low-alkaloid) and further comprises a chlorophyll level which is more than 80%, more than 85%, more than 90%, more than 95%, more than 100%, more than 105%, more than 110%, more than 115%, more than 120%, more than 125%, more than 130%, more than 135%, or more than 140% relative to the chlorophyll level of an unmodified control plant. In an aspect, a modified tobacco plant disclosed comprises a modification conferring a desired trait (e.g., low-nicotine, nicotine-free, or low-alkaloid) and further comprises a chlorophyll level which is between 70% and 140%, between 75% and 135%, between 80% and 130%, between 85% and 125%, between 90% and 120%, between 95% and 115%, or between 100% and 110% relative to the chlorophyll level of an unmodified control plant. In an aspect, a modified tobacco plant disclosed comprises a modification conferring a desired trait (e.g., low-nicotine, nicotine-free, or low-alkaloid) and further comprises a chlorophyll level which is between 70% and 80%, between 75% and 85%, between 80% and 90%, between 85% and 95%, between 90% and 100%, between 95% and 105%, between 105% and 115%, between 110% and 120%, between 115% to 125%, between 120% and 130%, between 125 and 135%, or between 130% and 140% relative to the chlorophyll level of an unmodified control plant.

In an aspect, a modified tobacco plant comprises a modification conferring a desired trait (e.g., low-nicotine, nicotine-free, or low-alkaloid) and further comprises a mesophyll cell number per unit leaf area which is more than 80%, more than 85%, more than 90%, more than 95%, more than 100%, more than 105%, more than 110%, more than 115%, more than 120%, more than 125%, more than 130%, more than 135%, or more than 140% relative to the mesophyll cell number per unit leaf area of an unmodified control plant. In an aspect, a modified tobacco plant disclosed comprises a modification conferring a desired trait (e.g., low-nicotine, nicotine-free, or low-alkaloid) and further comprises a mesophyll cell number per unit leaf area which is between 70% and 140%, between 75% and 135%, between 80% and 130%, between 85% and 125%, between 90% and 120%, between 95% and 115%, or between 100% and 110% relative to the mesophyll cell number per unit leaf area of an unmodified control plant. In an aspect, a modified tobacco plant disclosed comprises a modification conferring a desired trait (e.g., low-nicotine, nicotine-free, or low-alkaloid) and further comprises a mesophyll cell number per unit leaf area which is between 70% and 80%, between 75% and 85%, between 80% and 90%, between 85% and 95%, between 90% and 100%, between 95% and 105%, between 105% and 115%, between 110% and 120%, between 115% to 125%, between 120% and 130%, between 125 and 135%, or between 130% and 140% relative to the mesophyll cell number per unit leaf area of an unmodified control plant.

In an aspect, a low-nicotine or nicotine-free tobacco variety is adapted for machine harvesting. In another aspect, a low-nicotine or nicotine-free tobacco variety disclosed is harvested mechanically.

In an aspect, a method for improving leaf quality in a reduced-alkaloid tobacco plant is provided, the method comprising: growing a tobacco plant; reducing the level of putrescine in the tobacco plant, and harvesting leaves from the tobacco plant.

In an aspect, a method for improving leaf quality in a reduced-alkaloid tobacco plant is provided, the method comprising: growing a tobacco plant; suppressing the expression or activity of an ornithine decarboxylase (ODC) gene in the tobacco plant, and harvesting leaves from the tobacco plant. In one aspect, the suppressing step is within 2, 4, 6, or 8 WPT. In an aspect, the suppressing step comprises suppressing a ODC gene both prior to and after topping a tobacco plant. In one aspect, the suppressing step does not include the use of a chemical inhibitor. In an aspect, the suppressing step is accomplished by inducing the expression of a non-coding RNA for suppression of an ornithine decarboxylase (ODC) gene. In one aspect, the suppressing step comprises applying an ODC inhibitor to the tobacco plant. In an aspect, the suppressing is accomplished by applying an ODC inhibitor to a tobacco plant. In one aspect, an ODC inhibitor is DFMO.

In an aspect, tobacco plants provided are hybrid plants. Hybrids can be produced by preventing self-pollination of female parent plants (e.g., seed parents) of a first variety, permitting pollen from male parent plants of a second variety to fertilize the female parent plants, and allowing F1 hybrid seeds to form on the female plants. Self-pollination of female plants can be prevented by emasculating the flowers at an early stage of flower development. Alternatively, pollen formation can be prevented on the female parent plants using a form of male sterility. For example, male sterility can be produced by male sterility (MS), or transgenic male sterility where a transgene inhibits microsporogenesis and/or pollen formation, or self-incompatibility. Female parent plants containing MS are particularly useful. In aspects in which the female parent plants are MS, pollen may be harvested from male fertile plants and applied manually to the stigmas of MS female parent plants, and the resulting F1 seed is harvested.

Plants can be used to form single-cross tobacco F1 hybrids. Pollen from a male parent plant is manually transferred to an emasculated female parent plant or a female parent plant that is male sterile to form F1 seed. Alternatively, three-way crosses can be carried out where a single-cross F1 hybrid is used as a female parent and is crossed with a different male parent. As another alternative, double-cross hybrids can be created where the F1 progeny of two different single-crosses are themselves crossed. Self-incompatibility can be used to particular advantage to prevent self-pollination of female parents when forming a double-cross hybrid.

In an aspect, a low-nicotine or nicotine-free tobacco variety is male sterile. In another aspect, a low-nicotine or nicotine-free tobacco variety is cytoplasmic male sterile. Male sterile tobacco plants may be produced by any method known in the art. Methods of producing male sterile tobacco are described in Wernsman, E. A., and Rufty, R. C. 1987. Chapter Seventeen. Tobacco. Pages 669-698 In: Cultivar Development. Crop Species. W. H. Fehr (ed.), MacMillan Publishing Go., Inc., New York, N.Y. 761 pp.

In a further aspect, tobacco parts provided include, but are not limited to, a leaf, a stem, a root, a seed, a flower, pollen, an anther, an ovule, a pedicel, a fruit, a meristem, a cotyledon, a hypocotyl, a pod, an embryo, endosperm, an explant, a callus, a tissue culture, a shoot, a cell, and a protoplast. In an aspect, tobacco part provided does not include seed. In an aspect, this disclosure provides tobacco plant cells, tissues, and organs that are not reproductive material and do not mediate the natural reproduction of the plant. In another aspect, this disclosure also provides tobacco plant cells, tissues, and organs that are reproductive material and mediate the natural reproduction of the plant. In another aspect, this disclosure provides tobacco plant cells, tissues, and organs that cannot maintain themselves via photosynthesis. In another aspect, this disclosure provides somatic tobacco plant cells. Somatic cells, contrary to germline cells, do not mediate plant reproduction.

The provided cells, tissues and organs may be from seed, fruit, leaf, cotyledon, hypocotyl, meristem, embryos, endosperm, root, shoot, stem, pod, flower, inflorescence, stalk, pedicel, style, stigma, receptacle, petal, sepal, pollen, anther, filament, ovary, ovule, pericarp, phloem, vascular tissue. In another aspect, this disclosure provides a tobacco plant chloroplast. In a further aspect, this disclosure provides epidermal cells, stomata cell, leaf or root hairs, a storage root, or a tuber. In another aspect, this disclosure provides a tobacco protoplast.

Skilled artisans understand that tobacco plants naturally reproduce via seeds, not via asexual reproduction or vegetative propagation. In an aspect, this disclosure provides tobacco endosperm. In another aspect, this disclosure provides tobacco endosperm cells. In a further aspect, this disclosure provides a male or female sterile tobacco plant, which cannot reproduce without human intervention.

In an aspect, the present disclosure provides a nucleic acid molecule comprising at least about 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity to a sequence selected from the group consisting of SEQ ID NOs: 23-28, and fragments thereof. In an aspect, the present disclosure provides a polypeptide or protein comprising at least about 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% identity to an amino acid sequence selected from the group consisting of SEQ ID NOs: 29-34. In another aspect, the present disclosure provides a biologically active variant of a protein having an amino acid sequence selected from the group consisting of SEQ ID NOs: 29-34. A biologically active variant of a protein of the present disclosure may differ from that protein by as few as 1-15 amino acid residues, as few as 10, as few as 9, as few as 8, as few as 7, as few as 6, as few as 5, as few as 4, as few as 3, as few as 2, or as few as 1 amino acid residue. Also provided are orthologous genes or proteins of genes or proteins from the ODC pathway. “Orthologs” are genes derived from a common ancestral gene and which are found in different species as a result of speciation. Orthologs may share at least 60%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or greater sequence identity or similarity at the nucleotide sequence and/or the protein sequence level. Functions of orthologs are often highly conserved among species.

As used herein, the term “sequence identity” or “identity” in the context of two polynucleotides or polypeptide sequences makes reference to the residues in the two sequences that are the same when aligned for maximum correspondence over a specified comparison window. When percentage of sequence identity is used in reference to proteins it is recognized that residue positions which are not identical often differ by conservative amino acid substitutions, where amino acid residues are substituted for other amino acid residues with similar chemical properties (e.g., charge or hydrophobicity) and therefore do not change the functional properties of the molecule. When sequences differ in conservative substitutions, the percent sequence identity may be adjusted upwards to correct for the conservative nature of the substitution. Sequences that differ by such conservative substitutions are deemed to have “sequence similarity” or “similarity.”

Nucleic acid molecules, polypeptides, or proteins provided can be isolated or substantially purified. An “isolated” or “purified” nucleic acid molecule, polypeptide, protein, or biologically active portion thereof, is substantially or essentially free from components that normally accompany or interact with the polynucleotide or protein as found in its naturally occurring environment. For example, an isolated or purified polynucleotide or protein is substantially free of other cellular material, or culture medium when produced by recombinant techniques, or substantially free of chemical precursors or other chemicals when chemically synthesized.

The present disclosure further provides a method manufacturing a tobacco product comprising tobacco material from tobacco plants disclosed. In an aspect, methods comprise conditioning aged tobacco material made from tobacco plants to increase its moisture content from between about 12.5% and about 13.5% to about 21%, blending the conditioned tobacco material to produce a desirable blend. In an aspect, the method of manufacturing a tobacco product further comprises casing or flavoring the blend. Generally, during the casing process, casing or sauce materials are added to blends to enhance their quality by balancing the chemical composition and to develop certain desired flavor characteristics. Further details for the casing process can be found in Tobacco Production, Chemistry and Technology, Edited by L. Davis and M. Nielsen, Blackwell Science, 1999.

Tobacco material provided can be also processed using methods including, but not limited to, heat treatment (e.g., cooking, toasting), flavoring, enzyme treatment, expansion and/or curing. Both fermented and non-fermented tobaccos can be processed using these techniques. Examples of suitable processed tobaccos include dark air-cured, dark fire cured, burley, flue cured, and cigar filler or wrapper, as well as the products from the whole leaf stemming operation. In an aspect, tobacco fibers include up to 70% dark tobacco on a fresh weight basis. For example, tobacco can be conditioned by heating, sweating and/or pasteurizing steps as described in U.S. Publication Nos. 2004/0118422 or 2005/0178398.

Tobacco material provided can be subject to fermentation. Fermenting typically is characterized by high initial moisture content, heat generation, and a 10 to 20% loss of dry weight. See, e.g., U.S. Pat. Nos. 4,528,993; 4,660,577; 4,848,373; and 5,372,149. In addition to modifying the aroma of the leaf, fermentation can change either or both the color and texture of a leaf. Also during the fermentation process, evolution gases can be produced, oxygen can be taken up, the pH can change, and the amount of water retained can change. See, for example, U.S. Publication No. 2005/0178398 and Tso (1999, Chapter 1 in Tobacco, Production, Chemistry and Technology, Davis & Nielsen, eds., Blackwell Publishing, Oxford). Cured, or cured and fermented tobacco can be further processed (e.g., cut, expanded, blended, milled or comminuted) prior to incorporation into the oral product. The tobacco, in some cases, is long cut fermented cured moist tobacco having an oven volatiles content of between 48 and 50 weight percent prior to mixing with the copolymer and optionally flavorants and other additives.

In an aspect, tobacco material provided can be processed to a desired size. In an aspect, tobacco fibers can be processed to have an average fiber size of less than 200 micrometers. In an aspect, tobacco fibers are between 75 and 125 micrometers. In another aspect, tobacco fibers are processed to have a size of 75 micrometers or less. In an aspect, tobacco fibers include long cut tobacco, which can be cut or shredded into widths of about 10 cuts/inch up to about 110 cuts/inch and lengths of about 0.1 inches up to about 1 inch. Double cut tobacco fibers can have a range of particle sizes such that about 70% of the double cut tobacco fibers falls between the mesh sizes of −20 mesh and 80 mesh.

Tobacco material provided can be processed to have a total oven volatiles content of about 10% by weight or greater; about 20% by weight or greater; about 40% by weight or greater; about 15% by weight to about 25% by weight; about 20% by weight to about 30% by weight; about 30% by weight to about 50% by weight; about 45% by weight to about 65% by weight; or about 50% by weight to about 60% by weight. Those of skill in the art will appreciate that “moist” tobacco typically refers to tobacco that has an oven volatiles content of between about 40% by weight and about 60% by weight (e.g., about 45% by weight to about 55% by weight, or about 50% by weight). As used herein, “oven volatiles” are determined by calculating the percentage of weight loss for a sample after drying the sample in a pre-warmed forced draft oven at 110° C. for 3.25 hours. The oral product can have a different overall oven volatiles content than the oven volatiles content of the tobacco fibers used to make the oral product. The processing steps described can reduce or increase the oven volatiles content.

Having now generally described the disclosure, the same will be more readily understood through reference to the following examples that are provided by way of illustration, and are not intended to be limiting of the present disclosure, unless specified.

The following are exemplary embodiments of the present disclosure.

Embodiment 1. A tobacco plant comprising an inducible promoter operably linked to a transcribable DNA sequence encoding a non-coding RNA for suppression of an ornithine decarboxylase (ODC) gene.

Embodiment 2. The tobacco plant of Embodiment 1, wherein said tobacco plant comprises a mutation or a transgene conferring a reduced level of nicotine.

Embodiment 3. The tobacco plant of Embodiments 1 or 2, wherein said tobacco plant is a low-alkaloid tobacco plant.

Embodiment 4. The tobacco plant of any one of Embodiments 1-3, wherein said tobacco plant comprises a nic1 mutation, a nic2 mutation, or both.

Embodiment 5. The tobacco plant of any one of Embodiments 1-4, wherein said tobacco plant comprises a mutation in a gene or locus encoding a protein selected from the group consisting of aspartate oxidase, agmatine deiminase (AIC), arginase, diamine oxidase, arginine decarboxylase (ADC), methylputrescine oxidase (MPO), NADH dehydrogenase, ornithine decarboxylase (ODC), phosphoribosylanthranilate isomerase (PRAI), putrescine N-methyltransferase (PMT), quinolate phosphoribosyl transferase (QPT), S-adenosyl-methionine synthetase (SAMS), A622, NBB1, BBL, MYC2, Nic1, Nic2, ethylene response factor (ERF) transcription factor, nicotine uptake permease (NUP), and MATE transporter.

Embodiment 6. The tobacco plant of any one of Embodiments 1-5, wherein said tobacco plant comprises a mutation in a gene or locus encoding a protein selected from the group consisting of ERF32, ERF34, ERF39, ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168.

Embodiment 7. The tobacco plant of any one of Embodiments 1-6, wherein said tobacco plant comprises a transgene targeting and suppressing a gene encoding a protein selected from the group consisting of aspartate oxidase, agmatine deiminase (AIC), arginase, diamine oxidase, arginine decarboxylase (ADC), methylputrescine oxidase (MPO), NADH dehydrogenase, ornithine decarboxylase (ODC), phosphoribosylanthranilate isomerase (PRAI), putrescine N-methyltransferase (PMT), quinolate phosphoribosyl transferase (QPT), S-adenosyl-methionine synthetase (SAMS), A622, NBB1, BBL, MYC2, Nic1, Nic2, ethylene response factor (ERF) transcription factor, nicotine uptake permease (NUP), and MATE transporter.

Embodiment 8. The tobacco plant of any one of Embodiments 1-7, wherein said tobacco plant comprises a transgene targeting and suppressing a gene encoding a protein selected from the group consisting of ERF32, ERF34, ERF39, ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168.

Embodiment 9. The tobacco plant of any one of Embodiments 1-8, wherein said tobacco plant is capable of producing a leaf comprising a comparable level of one or more polyamines relative to a comparable leaf of a control plant not comprising said mutation or said transgene.

Embodiment 10. The tobacco plant of any one of Embodiments 1-9, wherein said comparable level is within 20%, 17.5%, 15%, 12.5%, 10%, 7.5%, 5%, 2.5%, or 1% of the level in said control.

Embodiment 11. The tobacco plant of any one of claims Embodiments 1-10, wherein said tobacco plant is capable of producing a leaf comprising a comparable chlorophyll level relative to a comparable leaf of a control plant not comprising said mutation or said transgene.

Embodiment 12. The tobacco plant of any one of Embodiments 1-11, wherein said comparable chlorophyll level is within 20%, 17.5%, 15%, 12.5%, 10%, 7.5%, 5%, 2.5%, or 1% of the level in said control.

Embodiment 13. The tobacco plant of any one of Embodiments 1-12, wherein said tobacco plant is capable of producing a leaf comprising a comparable number of mesophyll cell per unit of leaf area relative to a comparable leaf of a control plant not comprising said mutation or said transgene.

Embodiment 14. The tobacco plant of any one of Embodiments 1-13, wherein said comparable mesophyll cell per unit of leaf area is within 20%, 17.5%, 15%, 12.5%, 10%, 7.5%, 5%, 2.5%, or 1% of the level in said control.

Embodiment 15. The tobacco plant of any one of Embodiments 1-14, wherein said tobacco plant is capable of producing a leaf comprising a comparable epidermal cell size relative to a comparable leaf of a control plant not comprising said mutation or said transgene.

Embodiment 16. The tobacco plant of any one of Embodiments 1-15, wherein said comparable epidermal cell size is within 20%, 17.5%, 15%, 12.5%, 10%, 7.5%, 5%, 2.5%, or 1% of the level in said control.

Embodiment 17. The tobacco plant of any one of Embodiments 1-16, wherein said tobacco plant comprises a comparable leaf yield relative to a comparable leaf of a control plant not comprising said mutation or said transgene.

Embodiment 18. The tobacco plant of any one of Embodiments 1-17, wherein said comparable leaf yield is within 20%, 17.5%, 15%, 12.5%, 10%, 7.5%, 5%, 2.5%, or 1% of the level in said control.

Embodiment 19. The tobacco plant of any one of Embodiments 1-18, wherein said tobacco plant exhibits a comparable insect herbivory susceptibility relative to a comparable leaf of a control plant not comprising said mutation or said transgene.

Embodiment 20. The tobacco plant of any one of Embodiments 1-19, wherein said ornithine decarboxylase (ODC) gene encodes a polypeptide sequence having at least 80%, at least 85%, at least 90%, at least 95%, at least 97%, at least 99%, or 100% identity to a sequence selected from the group consisting of SEQ ID Nos: 29-34.

Embodiment 21. The tobacco plant of any one of Embodiments 1-20, wherein said ODC gene comprises a nucleotide sequence having at least 90%, at least 95%, at least 97%, at least 99%, or 100% identity to a sequence selected from the group consisting of SEQ ID Nos: 23-28.

Embodiment 22. The tobacco plant of any one of Embodiments 1-21, wherein said inducible promoter is a topping-inducible promoter.

Embodiment 23. The tobacco plant of any one of Embodiments 1-22, wherein said inducible promoter is also a tissue-specific or tissue-preferred promoter.

Embodiment 24. The tobacco plant of any one of Embodiments 1-23, wherein said tissue-specific or tissue-preferred promoter is specific or preferred for one or more tissues or organs selected from the group consisting of shoot, root, leaf, stem, flower, sucker, root tip, mesophyll cells, epidermal cells, and vasculature.

Embodiment 25. The tobacco plant of any one of Embodiments 1-24, wherein said inducible promoter regulates root specific or preferred expression.

Embodiment 26. The tobacco plant of any one of Embodiments 1-25, wherein said inducible promoter comprises a sequence selected from the group consisting of SEQ ID Nos: 1-11.

Embodiment 27. The tobacco plant of any one of Embodiments 1-26, wherein said inducible promoter regulates leaf specific or preferred expression.

Embodiment 28. The tobacco plant of any one of Embodiments 1-27, wherein said inducible promoter comprises a sequence selected from the group consisting of SEQ ID Nos: 12-21.

Embodiment 29. The tobacco plant of any one of Embodiments 1-28, wherein said inducible promoter is a heterologous to said transcribable DNA sequence.

Embodiment 30. The tobacco plant of any one of Embodiments 1-29, wherein said non-coding RNA is selected from the group consisting of microRNA (miRNA), anti-sense RNA, small interfering RNA (siRNA), a trans-acting siRNA (ta-siRNA), and hairpin RNA (hpRNA).

Embodiment 31. The tobacco plant of any one of Embodiments 1-30, wherein said non-coding RNA comprises a nucleotide sequence having at least 90%, at least 95%, at least 97%, at least 99%, or 100% identity to a sequence selected from the group consisting of SEQ ID Nos: 35 and 36.

Embodiment 32. The tobacco plant of any one of Embodiments 1-31, wherein said non-coding RNA is provided in an ODC RNAi construct comprising a nucleotide sequence having at least 90% identity to SEQ ID No: 22.

Embodiment 33. A tobacco plant, or part thereof, comprising a nic1 mutation, a nic2 mutation, or both, and further comprising a transgene or mutation providing an early-senescence trait.

Embodiment 34. The tobacco plant of Embodiment 33, wherein said mutation providing an early-senescence trait is yellow burley1 (-yb1).

Embodiment 35. The tobacco plant of Embodiments 33 or 34, wherein said mutation providing an early-senescence trait is yellow burley2 (-yb2).

Embodiment 36. The tobacco plant of any one of Embodiments 33-35, wherein said mutation providing an early-senescence trait is pale yellow (PY).

Embodiment 37. A tobacco plant, or part thereof, comprising relative to a control tobacco plant:

-   -   a. a first genome modification providing a lower level of         nicotine or total alkaloid, and     -   b. a second genome modification providing a comparable level of         one or more traits selected from the group consisting of         -   i. total leaf polyamine level,         -   ii. total root polyamine level,         -   iii. total leaf chlorophyll level,         -   iv. mesophyll cell number per leaf area unit, and         -   v. leaf epidermal cell size; and     -   wherein said control plant does not have both said first and         said second genome modifications.

Embodiment 38. A tobacco plant, or part thereof, comprising relative to a control tobacco plant:

-   -   a. a first genome modification providing a lower level of         nicotine or total alkaloid, and     -   b. a second genome modification providing a comparable level of         total leaf polyamine level, wherein said control plant does not         have both said first and said second genome modifications.

Embodiment 39. A tobacco plant, or part thereof, comprising relative to a control tobacco plant:

-   -   a. a first genome modification providing a lower level of         nicotine or total alkaloid, and     -   b. a second genome modification providing a comparable level of         total root polyamine level, wherein said control plant does not         have both said first and said second genome modifications.

Embodiment 40. A tobacco plant, or part thereof, comprising relative to a control tobacco plant:

-   -   a. a first genome modification providing a lower level of         nicotine or total alkaloid, and     -   b. a second genome modification providing a comparable level of         total leaf chlorophyll level, wherein said control plant does         not have both said first and said second genome modifications.

Embodiment 41. A tobacco plant, or part thereof, comprising relative to a control tobacco plant:

-   -   a. a first genome modification providing a lower level of         nicotine or total alkaloid, and     -   b. a second genome modification providing a comparable level of         mesophyll cell number per leaf area unit, wherein said control         plant does not have both said first and said second genome         modifications.

Embodiment 42. A tobacco plant, or part thereof, comprising relative to a control tobacco plant:

-   -   a. a first genome modification providing a lower level of         nicotine or total alkaloid, and     -   b. a second genome modification providing a comparable level of         leaf epidermal cell size, wherein said control plant does not         have both said first and said second genome modifications.

Embodiment 43. The tobacco plant, or part thereof, of any one of Embodiments 37-42, wherein said tobacco plant comprises a reduced amount of total conjugated polyamines in leaves relative to said control tobacco plant.

Embodiment 44. The tobacco plant, or part thereof, of any one of Embodiments 37-43, wherein said tobacco plant comprises a reduced amount of total conjugated polyamines in roots relative to said control tobacco plant.

Embodiment 45. The tobacco plant, or part thereof, of any one of Embodiments 37-44, wherein said tobacco plant comprises a reduced amount of total free polyamines in leaves relative to said control tobacco plant.

Embodiment 46. The tobacco plant, or part thereof, of any one of Embodiments 37-45, wherein said tobacco plant comprises a reduced amount of total conjugated polyamines in roots relative to said control tobacco plant.

Embodiment 47. The tobacco plant, or part thereof, of any one of Embodiments 37-46, wherein said tobacco plant comprises a reduced amount of total conjugated form of one or more polyamines selected from the group consisting of putrescine, spermidine and spermine in leaves relative to said control tobacco plant.

Embodiment 48. The tobacco plant, or part thereof, of any one of Embodiments 37-47, wherein said tobacco plant comprises a reduced amount of total conjugated form of one or more polyamines selected from the group consisting of putrescine, spermidine and spermine in roots relative to said control tobacco plant.

Embodiment 49. The tobacco plant, or part thereof, of any one of Embodiments 37-48, wherein said tobacco plant comprises a reduced amount of total free form of one or more polyamines selected from the group consisting of putrescine, spermidine and spermine in leaves relative to said control tobacco plant.

Embodiment 50. The tobacco plant, or part thereof, of any one of Embodiments 37-49, wherein said tobacco plant comprises a reduced amount of total conjugated form of one or more polyamines selected from the group consisting of putrescine, spermidine and spermine in roots relative to said control tobacco plant.

Embodiment 51. The tobacco plant, or part thereof, of any one of Embodiments 37-50, wherein said first genome modification provides a lower level of nicotine compared to said control tobacco plant.

Embodiment 52. The tobacco plant, or part thereof, of any one of Embodiments 37-51, said first genome modification, said second genome modification, or both comprise a transgene, a mutation, or both.

Embodiment 53. The tobacco plant, or part thereof, of any one of Embodiments 37-52, said first genome modification, said second genome modification, or both comprise a transgene.

Embodiment 54. The tobacco plant, or part thereof, of any one of Embodiments 37-53, said first genome modification, said second genome modification, or both comprise a mutation.

Embodiment 55. The tobacco plant, or part thereof, of any one of Embodiments 37-54, said first genome modification, said second genome modification, or both are not transgene-based.

Embodiment 56. The tobacco plant, or part thereof, of any one of Embodiments 37-55, said first genome modification, said second genome modification, or both are not mutation-based.

Embodiment 57. The tobacco plant, or part thereof, of any one of Embodiments 37-56, wherein said first genome modification comprises a nic1 mutation, a nic2 mutation, or both.

Embodiment 58. The tobacco plant, or part thereof, of any one of Embodiments 37-57, wherein said first genome modification comprises a transgene targeting the Nic1 locus, a transgene targeting the Nic2 locus, or both.

Embodiment 59. The tobacco plant, or part thereof, of any one of Embodiments 37-58, wherein said second genome modification comprises a transcribable DNA sequence encoding a non-coding RNA for suppression of an ornithine decarboxylase (ODC) gene, a MYB8 gene, or both.

Embodiment 60. The tobacco plant, or part thereof, of any one of Embodiments 37-59, wherein said a transcribable DNA sequence is operably linked to an heterologous promoter selected from the group consisting of a constitutive promoter, a developmental promoter, a tissue-specific promoter, a tissue-preferred promoter, an inducible promoter, and any combination thereof.

Embodiment 61. The tobacco plant, or part thereof, of any one of Embodiments 37-60, wherein said second genome modification comprises overexpression of an diamine oxidase, suppression of an arginine decarboxylase, or both.

Embodiment 62. The tobacco plant, or part thereof, of any one of Embodiments 37-61, wherein said first genome modification comprises a mutation in a gene or locus encoding a protein selected from the group consisting of aspartate oxidase, agmatine deiminase (AIC), arginase, diamine oxidase, arginine decarboxylase (ADC), methylputrescine oxidase (MPO), NADH dehydrogenase, ornithine decarboxylase (ODC), phosphoribosylanthranilate isomerase (PRAI), putrescine N-methyltransferase (PMT), quinolate phosphoribosyl transferase (QPT), and S-adenosyl-methionine synthetase (SAMS), A622, NBB1, BBL, MYC2, Nic1, Nic2, ethylene response factor (ERF) transcription factor, nicotine uptake permease (NUP), and MATE transporter.

Embodiment 63. The tobacco plant, or part thereof, of any one of Embodiments 37-62, wherein said first genome modification comprises a mutation in a gene or locus encoding a protein selected from the group consisting of ERF32, ERF34, ERF39, ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168.

Embodiment 64. The tobacco plant, or part thereof, of any one of Embodiments 37-63, wherein said first genome modification comprises a transgene targeting and suppressing a gene or locus encoding a protein selected from the group consisting of aspartate oxidase, agmatine deiminase (AIC), arginase, diamine oxidase, arginine decarboxylase (ADC), methylputrescine oxidase (MPO), NADH dehydrogenase, ornithine decarboxylase (ODC), phosphoribosylanthranilate isomerase (PRAI), putrescine N-methyltransferase (PMT), quinolate phosphoribosyl transferase (QPT), and S-adenosyl-methionine synthetase (SAMS), A622, NBB1, BBL, MYC2, Nic1, Nic2, ethylene response factor (ERF) transcription factor, nicotine uptake permease (NUP), and MATE transporter.

Embodiment 65. The tobacco plant, or part thereof, of any one of Embodiments 37-64, wherein said first genome modification comprises a transgene targeting and suppressing a gene or locus encoding a protein selected from the group consisting of ERF32, ERF34, ERF39, ERF189, ERF115, ERF221, ERF104, ERF179, ERF17, and ERF168.

Embodiment 66. The tobacco plant, or part thereof, of any one of Embodiments 37-65, wherein said lower level is measured at a time selected from the group consisting of immediately before flowering, at topping, 1 week-post-topping (WPT), 2 WPT, 3 WPT, 4 WPT, 5 WPT, 6 WPT, 7 WPT, 8 WPT, and at harvest.

Embodiment 67. The tobacco plant, or part thereof, of any one of Embodiments 37-66, wherein said comparable level is measured at a time selected from the group consisting of immediately before flowering, at topping, 1 week-post-topping (WPT), 2 WPT, 3 WPT, 4 WPT, 5 WPT, 6 WPT, 7 WPT, 8 WPT, and at harvest.

Embodiment 68. The tobacco plant, or part thereof, of any one of Embodiments 37-67, wherein said tobacco plant is capable of producing a leaf with a leaf grade comparable to that of a leaf from said control plant.

Embodiment 69. The tobacco plant, or part thereof, of any one of Embodiments 37-68, wherein said tobacco plant has a total leaf yield comparable to said control plant.

Embodiment 70. The tobacco plant, or part thereof, of any one of the preceding Embodiments, wherein said tobacco plant comprises a nicotine level selected from the group consisting of less than 3%, less than 2.75%, less than 2.5%, less than 2.25%, less than 2.0%, less than 1.75%, less than 1.5%, less than 1.25%, less than 1%, less than 0.9%, less than 0.8%, less than 0.7%, less than 0.6%, less than 0.5%, less than 0.4%, less than 0.3%, less than 0.2%, less than 0.1%, and less than 0.05%.

Embodiment 71. The tobacco plant, or part thereof, of any one of Embodiments 37-70, wherein said tobacco plant comprises nicotine at a level below 1%, below 2%, below 5%, below 8%, below 10%, below 12%, below 15%, below 20%, below 25%, below 30%, below 40%, below 50%, below 60%, below 70%, or below 80% of the nicotine level of said control plant when grown in comparable growth conditions.

Embodiment 72. A population of the tobacco plants of any one of the preceding Embodiments.

Embodiment 73. Cured tobacco material from the tobacco plant of any one of the preceding Embodiments.

Embodiment 74. The cured tobacco material of Embodiment 73, wherein said cured tobacco material is made by a curing process selected from the group consisting of flue curing, air curing, fire curing, and sun curing.

Embodiment 75. A tobacco blend comprising said cured tobacco material of Embodiments 73 or 74.

Embodiment 76. The tobacco blend of any one of Embodiments 73-75, wherein said cured tobacco material constitutes about at least 10%, at least 15%, at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, or at least 95% of cured tobacco in said tobacco blend by weight.

Embodiment 77. The tobacco blend of any one of Embodiments 73-76, wherein said cured tobacco material constitutes about at least 10%, at least 15%, at least 20%, at least 25%, at least 30%, at least 35%, at least 40%, at least 45%, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, or at least 95% of cured tobacco in said tobacco blend by volume.

Embodiment 78. A tobacco product comprising the cured tobacco material of any one of Embodiments 73-77.

Embodiment 79. The tobacco product of Embodiment 78, wherein said tobacco product is selected from the group consisting of a cigarette, a cigarillo, a non-ventilated recess filter cigarette, a vented recess filter cigarette, a cigar, snuff, pipe tobacco, cigar tobacco, cigarette tobacco, chewing tobacco, leaf tobacco, shredded tobacco, and cut tobacco.

Embodiment 80. The tobacco product of Embodiments 78 or 79, wherein said tobacco product is selected from the group consisting of loose leaf chewing tobacco, plug chewing tobacco, moist snuff, and nasal snuff.

Embodiment 81. A method for improving leaf quality in a reduced-alkaloid tobacco plant, said method comprising:

-   -   a. Growing a tobacco plant;     -   b. Reducing the level of putrescine in said tobacco plant,     -   c. Harvesting leaves from said tobacco plant.

Embodiment 82. A method for improving leaf quality in a reduced-alkaloid tobacco plant, said method comprising:

-   -   a. Growing a tobacco plant;     -   b. Suppressing the expression or activity of an ornithine         decarboxylase (ODC) gene in said tobacco plant,     -   c. Harvesting leaves from said tobacco plant.

Embodiment 83. The method of Embodiment 81 or 82, wherein said suppressing is within 2, 4, 6, or 8 WPT.

Embodiment 84. The method of any one of Embodiments 81-83, wherein said suppressing comprises suppressing said ODC gene both prior to and after topping said tobacco plant.

Embodiment 85. The method of any one of Embodiments 82-84, wherein said suppressing does not include the use of a chemical inhibitor.

Embodiment 86. The method of any one of Embodiments 82-85, wherein said suppressing comprises applying an ODC inhibitor to said tobacco plant.

Embodiment 87. The method of any one of Embodiments 82-86, wherein said suppressing is by applying an ODC inhibitor to said tobacco plant.

Embodiment 88. The method of any one of Embodiments 82-87, wherein said ODC inhibitor is DFMO.

Embodiment 89. The method of any one of Embodiments 82-88, wherein said suppressing is by inducing the expression of a non-coding RNA for suppression of said ornithine decarboxylase (ODC) gene.

Embodiment 90. The method of any one of Embodiments 82-89, wherein said method further comprises reducing nitrogen fertilization or reducing nitrate.

EXAMPLES Example 1: Plant Material and General Growth Conditions

Seeds of Nicotiana tabacum L. cv. Burley 21 wild-type NA, as well as HI (nic2), LI (nic1) and LA (nic1nic2) near-isogenic varieties were obtained from the US Nicotiana Germplasm Collection at North Carolina State University and used in all greenhouse experiments. Seeds were germinated in pots under greenhouse conditions at 27/23° C. day/night temperature and a 16-h photoperiod (˜200 mmol s⁻¹ m⁻²; λ=400-700 nm) at 70% relative humidity. Five week-old tobacco plantlets were transferred to 13-L pots with standard substrate (Einheitserde, Fröndenberg, Germany). The plants were attached to a continuous drip irrigation system active every 4 h for ˜5 min and were irrigated with 0.7% (w/v) Ferty 2 Mega containing 16% nitrogen, (Planta Düngemittel, Regenstauf, Germany) for 5 min h⁻¹ during the 16-h photoperiod and grown for 4 additional weeks. Greenhouse plants were topped when 50% of the plants had at least one open flower. After topping, plants grew for additional 4-7 weeks until harvest, 30 days post-topping. Tobacco leaves used for polyamine analysis were collected from greenhouse-grown plants at three time points: before flowering (6.5-week-old plants), just before topping, i.e. the removal of the floral apex (9-week-old plants) and at harvest (13-week-old plants, 4 weeks post-topping). Root samples were collected at topping and harvest. LA and NA tobacco plants were grown in the field under 135 units of nitrogen per acre and sampled at 1 week post-topping for polyamine analysis.

For treatment with polyamine biosynthesis inhibitors, 5 mM D-Arginine (AKos, Steinen, Germany), 2 mM DFMO (Synchem Ug & Co. KG, Felsberg, Germany) alone or in combination with 0.5 mM Ethephon®, or 0.5 mM Ethephon® alone (Merck KGaA, Darmstadt, Germany), were diluted in the same amount of water used for daily irrigation and applied to LA plants every 4 hours at 9 am, 12 am, 3 pm and 6 pm three times per week instead of the drip irrigation system. The treatment with D-arginine and DFMO started before flowering (˜2.5 weeks before topping when plants were still in the vegetative growth stage) for a period of 6 weeks until harvest, whereas Ethephon® was applied from topping to harvest (4 weeks in total) to avoid early senescence in the LA plants. Twelve plants per inhibitor were treated. NA plants were used as controls and treated in the same way as the LA plants. More description of experimental procedures and data can be found at Nölke G, et al. Polyamines delay leaf maturation in low-alkaloid tobacco varieties. Plant Direct. 2018; 2:1-12.

Example 2: Chlorophyll Measurements

Chlorophyll contents were determined by measuring leaf absorbance in the red and infrared regions using a SPAD-502 Plus device (Minolta Camera Co., Osaka, Japan). Chlorophyll was measured twice in the same day at different positions in all fully expanded (length>15 cm) leaves (leaves 6-26) from six randomly-selected plants from each line at five growth stages: before flowering (6.5-week-old plants and 2.5 weeks before topping), at topping, 1 and 2.5 WPT and at harvest (30 days post topping) before flowering. The total chlorophyll content was calculated as an average of all measured leaf chlorophyll values per plant to minimize the influence of leaf position.

Example 3: Leaf Cell Microscopy

Four leaf discs (1 cm²) cut form leaf 15 from six biological replicates at different development stages (before flowering, at topping, 1 WPT and at harvest) were mounted on slides and imaged using a Leica DM R microscope (Leica, Wetzlar, Germany) with a 10× air objective. Images were imported into ImageJ and Adobe Photoshop CS5.1 software and the cells per unit area were counted using Count Tool in the Photoshop CS5. A standard area was designated to use for cell counting three times across all images and care was taken to avoid counting any cell twice.

Example 4: Determination of ODC and ADC Activities

To determine enzymatic activities, 500 mg of tobacco leaf tissue collected from leaf 23 of three biological replicates of was ground in 1 ml HEPES extraction buffer (100 mM HEPES, 2 mM dithiothreitol (DTT), 1 mM EDTA, pH 7.5) and 100 mg of polyvinylpyrrolidone was added during grinding. Following centrifugation (13,000 g, 10 min, 4° C.), the enzyme activities were measured using an isotopic method as described by Capell et al. (1998) by measuring the release of ¹⁴CO2. L-[1-¹⁴C]Arg and L-[1-¹⁴C]Orn were used as radioactive substrates.

Example 5: Polyamine Extraction and Analysis

For polyamine analysis, 150 μg of leaf or root material was harvested from plants grown in the greenhouse at different stages of development: before flowering (leaves 6 and 12, numbered from base), at topping (leaves 19 and 23 and roots) and at harvest (leaves 23 and 24 and roots). Samples were collected after 4 h of illumination from three biological replicates and were flash frozen in liquid nitrogen. For field-grown plants, leaf material was collected from five well-expanded upper leaves from three biological replicates. Plant material was ground in 1.6 ml pre-chilled 10% (v/v) perchloric acid and incubated at 4° C. for 1 h. The extract was vortexed for 10 s and centrifuged (16,000 g, 15 min, 4° C.) before 800 μl of the supernatant was mixed with 100 μl 1 mM hexamethylenediamine. Then, 10 μl of the clear supernatant was transferred to a fresh 2-ml tube and polyamines were extracted with 200 μl of cyclohexane for the dansilation of free polyamines. For the extraction of conjugated polyamines, the pellet was resuspended in 1600 μl 1 M NaOH and 200 μl 1 mM hexamethylenediamine and centrifuged as above. The clear supernatant (200 μl) was transferred to a 2-ml glass ampule containing 12 M HCl, mixed and incubated for 16 h at 110° C. overnight for the hydrolysis of conjugated polyamines. The dansilation of free and conjugated polyamines was carried out with dansyl chloride as described by Flores and Galston (1982).

The dansylated polyamines were measured by LC-MS/MS. All experiments were carried out on a 3200 QTRAP™ mass spectrometer (Sciex, Darmstadt, Germany) coupled to an HPLC Agilent 1200 system (Waldbronn, Germany). The mass spectrometer was equipped with an electrospray ionization source. The sample was separated on a reversed-phase Synergi Fusion with 80 Å pore size, 4 μm particle size and dimensions of 50 mm×2.0 mm internal diameter (Phenomenex, Aschaffenburg, Germany) with the corresponding guard column at a flow rate of 800 μl/min. The column oven was heated to 30° C. For elution, solvent A comprised 94.9% (v/v) water, 5% (v/v) acetonitrile, 0.1% (v/v) formic acid and solvent B comprised 94.9% (v/v) acetonitrile, 5% (v/v) water, 0.1% (v/v) formic acid. The elution following elution profiles was used: 1 min, hold at 60% solvent A/40% solvent B; 3 min, linear increase to 100% solvent B, 3 min hold at 100% solvent B; rapid linear decrease to 60% solvent A/40% solvent B in 0.1 min; hold for 1 min. The total run time was 8 min and the sample volume injected in each run was 10 μl.

The mass spectrometer was set to unit resolution in Q1 and Q3. All measurements were captured in multiple reaction monitoring mode. For compound optimization, standards were prepared according to the dansylation protocol, diluted in 50:50 (v/v) methanol/water and infused with a flow rate of 10 μl/min with the syringe pump directly connected to the ion source. Declustering potential, collision energy, collision cell entrance potential, collision cell exit potential and entrance potential were optimized for all compounds using automated compound optimization (Table 3). The ion source parameters were set to: capillary voltage=5.5 kV, heater gas temperature=500° C., curtain gas=30 psi, nebulizing gas=70 psi, drying gas=70 psi, and collision gas=medium. For each analyte, one transition was used for quantification and another as a qualifier. The acquired data was processed using Analyst v1.6 (Sciex). The mass calibration of the 3200 QTRAP was achieved using polypropylene glycol standards (Standards Chemical Kit with Low/High Concentration PPGs, Sciex) according to the manufacturer's instructions.

TABLE 3 Compound parameter for polyamine quantification. DP = declustering potential, CE = collision energy, CEP = collision cell entrance potential, CXP = collision cell exit potential, EP = entrance potential. Dansylated Quantifier/ Parent Product DP EP CEP CE CXP RT Compound Qualifier mass [m/z] mass [m/z] [eV] [eV] [eV] [eV] [eV] [min] Spermine Quantifier 1135.39 360.3 86 10 48 65 4 4.3 Spermine Qualifier 1135.39 170.3 86 10 48 121 4 4.3 Spermidine Quantifier 845.228 360.3 96 9.5 34 53 4 3.9 Spermidine Qualifier 845.228 170.3 96 9.5 34 81 4 3.9 Putrescine Quantifier 555.119 170.3 61 7.5 24 45 4 3.2 Putrescine Qualifier 555.119 168.3 61 7.5 24 79 4 3.2 Hexamethyldiamine Quantifier 583.14 170.3 70 10 28.809 50 4 3.5 Hexamethyldiamine Qualifier 583.14 169.2 70 10 28.809 50 4 3.5

Example 6: Statistical Analysis

Differences between the genotypes were determined by applying one-way analysis of variance (ANOVA) followed by post-hoc Bonferroni test using Excel software (Microsoft, Redmond, Wash., USA). Two-tailed t-tests were applied. A p-value<0.05 was considered statistically significant.

Example 7: Biochemical and Morphological Differences Among the Four Varieties During Leaf Ripening

Progression of senescence in the Burley 21 NA, HI, LI and LA lines was monitored by measuring the loss of chlorophyll a and b in the leaves. The chlorophyll levels had declined significantly (p<0.01) in all genotypes after 1 week post-topping (WPT) (FIG. 1A). However, the leaves of the LI and LA plants contained significantly (p<0.001) higher levels of chlorophyll than the NA controls at 2.5 WPT (22% more in both genotypes) and at harvest (36% and 44% more in the LI and LA leaves, respectively), indicating slower chlorophyll degradation compared to NA controls. Loss of chlorophyll was correlated with morphological changes in the leaves of NA plants, i.e. they became wrinkly and leathery with yellow patches, whereas the LA leaves remained smooth, shiny and green (FIG. 1B).

Given the distinct leaf morphology in the LA and NA lines, the size and shape of the mesophyll cells were investigated at different time points. Before flowering, leaf 15 (numbered from the base) of the LA plants had smaller and more abundant mesophyll cells (more cells per unit leaf area) compared to the NA plants (FIG. 1C/D). From that time point until harvest, the number of leaf mesophyll cells per unit area declined at a similar rate in both the NA and LA lines, but the LA plants retained a significantly (p<0.05) greater number of mesophyll cells throughout ripening. The greatest difference in mesophyll cell number per unit area (54% more cells in the LA plants compared to NA controls) was observed at earlier stages of leaf development (before flowering). LI plants also contained more mesophyll cells than the NA plants but not to the degree observed in the LA plants, and there was no significant difference in mesophyll cell number between the HI and NA lines (data not shown).

Example 8: LA Plants Accumulate Higher Levels of Polyamines than NA Plants

To investigate the impact of the nic1nic2 double mutation on polyamine biosynthesis, the levels of free and conjugated putrescine, spermidine and spermine in the NA and LA plants were compared by liquid chromatography tandem mass spectrometry (LC-MS/MS). First, the polyamine content were analyzed in leaves 16-18 of field-grown plants. At 1 WPT, the total polyamine content was significantly higher (1.9-fold, p<0.001) in the LA plants compared to the NA plants (FIG. 2A). Compositional analysis revealed significantly higher levels of free putrescine (1.4-fold, p<0.05), conjugated putrescine (2.3-fold, p<0.005) and conjugated spermidine (1.9-fold, (p<0.005) levels in the leaves of the LA plants, indicating that the polyamine biosynthesis pathway is strongly induced by the nic1nic2 double mutant or that the inability of the substrates to be further processed into nicotine results in a buildup of these materials. In contrast, the level of free spermidine in the LA plants was lower than in the NA plants, although the difference was not statistically significant (p>0.05).

To minimize the effect of variable environmental factors on polyamine biosynthesis, further experiments were performed under controlled greenhouse conditions mirroring the average field conditions in terms of temperature, light and humidity (data not shown). The phenotypes of the NA and LA plants in the greenhouse at harvest (30 days post-topping) were similar to their counterparts grown in the field in terms of plant height, leaf number and leaf morphology (data not shown). The impact of wounding on polyamine biosynthesis was minimized by designing the experiments so that each leaf/root sample was collected only once per plant and time point. Time-course monitoring of the total polyamine content in leaves at the same developmental stage—i.e. leaf 12 before flowering, leaf 19 at topping and leaf 24 at harvest—revealed significantly (p<0.05) higher levels of polyamines in the LA plants before flowering (1.5-fold) and at harvest (2.1-fold) compared to the NA controls (FIG. 2B). The LA plants also accumulated significantly (p<0.05) higher levels of total polyamines in the roots at topping (2.4-fold) and at harvest (1.4-fold) compared to the NA controls (FIG. 2B)

Example 9: Effect of the Nic1nic2 Double Mutation on Polyamine Biosynthesis

Comparative analysis of the polyamine composition in selected leaves (leaf 6 before flowering, young leaf 23 at topping and mature leaf 23 at harvest) in the four varieties revealed that, before flowering, lines LI and LA contained significantly (p<0.05) higher levels of free putrescine than the NA controls (1.6-fold and 4.2-fold higher, respectively) and even higher levels of conjugated putrescine (2.1-fold and 5-fold higher, respectively) (FIG. 3A). The conjugated putrescine and spermidine fractions increased continuously during ripening in all four varieties, but remained significantly higher in LI and LA plants compared to NA controls (FIG. 3A). The greatest differential in polyamine content was observed in the LA leaves at harvest, with a 2.1-fold increase in the level of total polyamines compared to NA controls, including a 1.8-fold increase in free putrescine, a 2.9-fold increase in conjugated putrescine and a 2.4-fold increase in conjugated spermidine. However, there was no significant difference between the NA and HI varieties, indicating that the nic2 single mutation had a lower impact on polyamine accumulation.

At topping, the roots of the LA plants contained significantly (p<0.05) higher levels of free putrescine, conjugated putrescine and conjugated spermidine than the NA plants (2.6-fold, 2.9-fold and 2.5-fold increases, respectively) and such differences were also observed at harvest (1.6-fold, 1.4-fold and 2.5-fold increases, respectively) (FIG. 3B).

Example 10: The Polyamine Biosynthesis Pathway is More Active in the LA Plants

The relative contribution of ADC and ODC to putrescine biosynthesis was evaluated by measuring the activity of each enzyme in the leaves (leaf 23) and roots of the NA and LA plants at topping and harvest. ADC and ODC activity varied in an organ-specific and developmental stage-specific manner in both lines (FIG. 4). Whereas ADC activity was high in the leaves but minimal in the roots of both lines, ODC activity was higher in the younger leaves and roots, indicating that ODC is mainly responsible for putrescine biosynthesis in the roots. ADC activity was significantly higher (1.4-fold, p<0.05) in the leaves of the LA plants compared to the NA controls at topping and harvest (FIG. 4A). Similarly, ODC activity was significantly higher (p<0.05) in the LA plants compared to the NA controls in the roots at topping (1.8-fold) and at harvest (1.7-fold), and in young leaves at topping (1.5-fold) (FIG. 4B).

Example 11: Inhibition of Polyamine Biosynthesis in the LA Variety

Given the correlation between the higher polyamine levels in the LA variety and the undesirable leaf morphology, the effect of treating the plants with chemicals that inhibit ADC and ODC was evaluated. Preliminary experiments defined the appropriate inhibitor concentration, application time, treatment intensity and duration (data not shown). The levels of free and conjugated putrescine were significantly higher in the LA plants than the NA controls before flowering and at harvest (FIG. 3), so the ADC inhibitor D-arginine and the ODC inhibitor difluoromethylornithine (DFMO) were applied beginning 2.5 weeks before topping and continued the treatment until harvest. In addition, the plant growth regulator Ethephon® was used alone or in combination with DFMO to accelerate ripening via the liberation of ethylene. To avoid the early induction of senescence, Ethephon® was applied from topping until harvest.

The DFMO and DFMO/Ethephon® treatments achieved a partial amelioration of the morphological phenotype, such that the leaves of the LA plants took on some of the characteristics of the NA leaves (wrinkling and chlorophyll degradation), whereas treatment with Ethephon® alone reduced the chlorophyll content but did not affect leaf morphology (FIG. 5). Starting the DFMO treatment before flowering resulted in growth arrest, which was not observed when the treatment was started at topping (data not shown). The D-arginine treatment had no effect on the chlorophyll level or morphology of the LA plants.

The analysis of polyamine levels revealed that the DFMO treatment 2.5 weeks before topping increased the levels of total polyamines in the LA leaves by 2.1-fold, mainly reflecting higher levels of conjugated putrescine and conjugated spermidine (FIG. 6A). This higher proportion of conjugated polyamines remained until harvest in the plants treated with DFMO and DFMO/Ethephon®. In contrast, the treatment with Ethephon® alone led to a significant reduction in total polyamine levels at harvest, mainly reflecting the reduction of free and conjugated putrescine. In the roots, the DFMO treatment significantly reduced (p<0.05) the total polyamine content of the LA plants at topping (1.5-fold) and at harvest (1.4-fold) due mainly to reduction of free and conjugated putrescine and conjugated spermidine (FIG. 6B). This decrease was not reversed by the addition of Ethephon®. In contrast to the effect in leaves, the application of Ethephon® alone had no effect on the polyamine content of the roots. The D-arginine treatment had no effect on the polyamine content of the LA plants. The loss of polyamines in the roots could therefore reflect the inhibition of ODC activity, the main enzyme responsible for putrescine biosynthesis in roots.

Example 12: Alteration of Polyamine Levels by Genetic Engineering

Modified tobacco plants are made to suppress ODC activity in a nic1 nic2 mutant background. A topping-responsive promoter (e.g., SED ID Nos: 1 to 21) is used to drive an ODC RNAi cassette (e.g., SEQ ID No: 22) to achieve the suppression of one or more ODC genes (e.g., coding sequences or protein sequences shown in SEQ ID Nos: 23 to 34). Transgenic plants are generated and assessed for leaf phenotypes, including for example, total leaf polyamine level, total root polyamine level, total leaf chlorophyll level, mesophyll cell number per leaf area unit, leaf epidermal cell size, and cured leaf grade.

Modified tobacco plants are also made to modulate the expression and activity of a MYB8 gene in a nic1 nic2 mutant Burley background. MYB8 was reported to control inducible phenolamide levels by activating three hydroxycinnamoyl-coenzyme A:polyamine transferases in Nicotiana attenuata. See Onkokesung et al., Plant Physiology 158 (1) 389-407 (2012). A constitutive promoter or a topping-responsive promoter (e.g., SED ID Nos: 1 to 21) is used to drive an MYB8 RNAi cassette or an MYB8 cDNA sequence to achieve suppression or overexpression, respectively. Transgenic plants are generated and assessed for leaf phenotypes, including for example, total leaf polyamine level, total root polyamine level, total leaf chlorophyll level, mesophyll cell number per leaf area unit, leaf epidermal cell size, and cured leaf grade.

Example 13: A Breeding Population

Low-alkaloid tobacco hybrids, varieties, or lines can be made as a Burley type, a dark type, a flue-cured type, a Maryland type or an Oriental type tobacco, or can be essentially derived from BU 64, CC 101, CC 200, CC 27, CC 301, CC 400, CC 500, CC 600, CC 700, CC 800, CC 900, Coker 176, Coker 319, Coker 371 Gold, Coker 48, CU 263, DF911, Galpao tobacco, GL 26H, GL 350, GL 600, GL 737, GL 939, GL 973, HB 04P, K 149, K 326, K 346, K 358, K394, K 399, K 730, KDH 959, KT 200, KT204LC, KY 10, KY 14, KY 160, KY 17, KY 171, KY 907, KY907LC, KTY14×L8 LC, Little Crittenden, McNair 373, McNair 944, msKY 14×L8, Narrow Leaf Madole, NC 100, NC 102, NC 2000, NC 291, NC 297, NC 299, NC 3, NC 4, NC 5, NC 6, NC7, NC 606, NC 71, NC 72, NC 810, NC BH 129, NC 2002, Neal Smith Madole, OXFORD 207, ‘Perique’ tobacco, PVH03, PVH09, PVH19, PVH50, PVH51, R 610, R 630, R 7-11, R 7-12, RG 17, RG 81, RG H51, RGH 4, RGH 51, RS 1410, Speight 168, Speight 172, Speight 179, Speight 210, Speight 220, Speight 225, Speight 227, Speight 234, Speight G-28, Speight G-70, Speight H-6, Speight H20, Speight NF3, TI 1406, TI 1269, TN 86, TN86LC, TN 90, TN 97, TN97LC, TN D94, TN D950, TR (Tom Rosson) Madole, VA 309, or VA359, Maryland 609, HB3307PLC, HB4488PLC, KT206LC, KT209LC, KT210LC, KT212LC, R610LC, PVH2310, NC196, KTD14LC, KTD6LC, KTD8LC, PD7302LC, PD7305LC, PD7309LC, PD7318LC, PD7319LC, PD7312LC, ShireyLC, or any commercial tobacco variety according to standard tobacco breeding techniques known in the art. 

1-20. (canceled)
 21. A recombinant DNA construct comprising a topping inducible promoter operably linked to a transcribable DNA sequence encoding a non-coding RNA for suppression of an ornithine decarboxylase (ODC) gene having at least 90% identity to a sequence selected from the group consisting of SEQ ID NOs: 23, 24, 25, 26, 27, and
 28. 22. The recombinant DNA construct of claim 1, wherein said ornithine decarboxylase (ODC) gene encodes a polypeptide sequence having at least 90% identity to a sequence selected from the group consisting of SEQ ID NOs: 29, 30, 31, 32, 33, and
 34. 23. The recombinant DNA construct of claim 27, wherein said ODC gene comprises a nucleotide sequence having at least 95% identity to a sequence selected from the group consisting of SEQ ID NOs: 23, 24, 25, 26, 27, and
 28. 24. The recombinant DNA construct of claim 21, wherein said inducible promoter comprises a sequence selected from the group consisting of SEQ ID NOs: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, and
 21. 25. The recombinant DNA construct of claim 21, wherein said non-coding RNA comprises a nucleotide sequence having at least 95% identity to a sequence selected from the group consisting of SEQ ID NOs: 35 and
 36. 26. The recombinant DNA construct of claim 21, wherein said non-coding RNA is provided in an ODC RNAi construct comprising a nucleotide sequence having at least 95% identity to SEQ ID NO:
 22. 27. The recombinant DNA construct of claim 21, wherein said non-coding RNA is provided in an ODC RNAi construct comprising a nucleotide sequence having at least 99% identity to SEQ ID NO:
 22. 28. The recombinant DNA construct of claim 21, wherein said ornithine decarboxylase (ODC) gene encodes a polypeptide sequence having at least 95% identity to a sequence selected from the group consisting of SEQ ID NOs: 29, 30, 31, 32, 33, and
 34. 29. The recombinant DNA construct of claim 7, wherein said ODC gene comprises a nucleotide sequence having at least 99% identity to a sequence selected from the group consisting of SEQ ID NOs: 23, 24, 25, 26, 27, and
 28. 30. The recombinant DNA construct of claim 21, wherein said non-coding RNA comprises a nucleotide sequence having at least 99% identity to a sequence selected from the group consisting of SEQ ID NOs: 35 and
 36. 31. A recombinant DNA construct comprising a topping inducible promoter having at least 95% identity to a sequence selected from the group consisting of SEQ ID NOs: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, and 21, operably linked to a transcribable DNA sequence encoding a non-coding RNA for suppression of an ornithine decarboxylase (ODC) gene having at least 90% identity to a sequence selected from the group consisting of SEQ ID NOs: 23, 24, 25, 26, 27, and
 28. 32. The recombinant DNA construct of claim 1, wherein said ornithine decarboxylase (ODC) gene encodes a polypeptide sequence having at least 90% identity to a sequence selected from the group consisting of SEQ ID NOs: 29, 30, 31, 32, 33, and
 34. 33. The recombinant DNA construct of claim 27, wherein said ODC gene comprises a nucleotide sequence having at least 95% identity to a sequence selected from the group consisting of SEQ ID NOs: 23, 24, 25, 26, 27, and
 28. 34. The recombinant DNA construct of claim 21, wherein said non-coding RNA comprises a nucleotide sequence having at least 95% identity to a sequence selected from the group consisting of SEQ ID NOs: 35 and
 36. 35. The recombinant DNA construct of claim 21, wherein said ornithine decarboxylase (ODC) gene encodes a polypeptide sequence having at least 95% identity to a sequence selected from the group consisting of SEQ ID NOs: 29, 30, 31, 32, 33, and
 34. 36. The recombinant DNA construct of claim 7, wherein said ODC gene comprises a nucleotide sequence having at least 99% identity to a sequence selected from the group consisting of SEQ ID NOs: 23, 24, 25, 26, 27, and
 28. 37. The recombinant DNA construct of claim 21, wherein said non-coding RNA comprises a nucleotide sequence having at least 99% identity to a sequence selected from the group consisting of SEQ ID NOs: 35 and
 36. 